Great question...
since I have listened to the audios before I actually saw their respective Doctors on TV (BF is what made me get the classic Who DVDs).
Edit- oups.
Sorry, I wrote much too much on the topic.
I apologize in advance for the wall of text.
Let's start with Peter Davison... well, when I was a kid I liked All creatures great and small (as opposed to Doctor Who, this was actually dubbed and broadcast in German TV) and some episodes made a huge impact on me and might have been part of my later career decision (scientist looking at animal diseases). Also, when I studied back in the day, my whole study group did a complete re-watch of the series and one of my friends as a huge fan had the original English versions (as opposed to the dubbed German ones).
So when I heard Spare parts and Kingmaker and Church and the Crown, I just pictured Tristan Farnon from the series...
Sylvester was a bit harder, since I had only seen the Hobbit (and you barely recognize him as Radagast, it actually took a while to click with me that they are the same actor).
From listening to him, I did not imagine all the facial expressions, little ticks and movements he does on TV.
Plus, my first audios with him were the New Adventures of Berncie Summerfield. I found the stoned Doctor in the very first adventure highly annoying, so I was put off the 7th Doctor for a long while. Until last year, I basically avoided him. Only just recently I discovered my love for him and admire some of his great audio adventures (Live 34! Night Thoughts!) and after that, I even got some of his TV episodes.
Same with Ace- I am not sure what I pictured her as... in the New Adventures of Bernice Summerfield maybe with tattered Time Lord robes. Only very much later was I surprised about all that bomber jacket and Baseball bat thing. And it finally clicked with me what all this talk about explosives was about. Ace was pretty much a confusing mystery character to me- a human time lord? Who leaves explosives lying around? How is this coming together? This does NOT make any sense!
Yeah, maybe one should not start a story at the very end.
But I got there in the end.
I very much loved Remembrance of the Daleks and of course Survival. Both were absolutely awesome. I especially loved the over the top acting in Survival.
I still do not like Curse of Fenric. I thought that was very confusing and I think I might have to watch it again to try to figure out what is going on (and I did watch the supposedly less confusing directors cut).
Then there is Colin Baker.
I loved his audios from the start, he immediately clicked with me.
Funnily enough, when listening, I was not able to imagine that coat. So I think I imagined him with some type of dark suit.
Except for the Two Doctors, which I absolutely love, I avoided getting his TV episodes. The 6th Doctor was the very last one I finally made myself buying the DVDs from- just because there seems to be a consensus that his TV work is "unwatchable".
Impressionable as I am, of course I believed what people were saying.
And what can I say?
Colin is great.
There is also said that the 6th Doctor on audio and on TV are so glaringly different.
Now here is the funny thing- since I came from the audios first, when watching the Two Doctors... his rude behavior with Peri did not register with me at first, I just took it for him teasing and joking with Perry. I think the glaring differences and what he really says and does only clicked with me on a second watching.
Also, the reason I got the Two Doctors in the first place was because I loved the 2nd Doctor and Jamie so much- I did not get it for Colin at all.
Well, that was a pleasant surprise.
To this day, I cannot really explain why I like the Two Doctors so much. It is completely non-sensical from a story perspective. There are plenty of head-shaking moments. Some of the characters and costumes and text are beyond weird. And all that over-acting...
But the actors had such fun doing this, and I got the impression they knew how ridiculous everything was, but they played it with such conviction anyways... somehow the good vibes are infectious.
From the moment I watched this, when I listened to a 6th Doctor audio, I pictured him in that colourful waistcoat.
I am still not able to imagine Colin in his real coat when listening to an audio. I think my brain is simply not able to compute this.
Finally, we have McGann.
I started out my BF career pretty much with New Series stuff (although, since I am a Warhammer fan, my very first BF releases were the Warhammer stuff they did. This was before I became a Whovian). Random searches online revealed to me the Diary of River Song. Since I liked BFs work on the Warhammer stuff, I thought I give it a try. So McGann was very much my first audio Doctor.
I liked it so much that I had a look for what people said were his best audios and got Dark Eyes (among a few others).
And OMG, Dark Eyes was amazing and a real eye opener. This is how I became a Big Finish fan.
I basically went from there.
This must also have been the time I found the clip of the Night of the Doctor on youtube, and that was pretty much my 8th Doctor, right there, like in the audios.
I only ever imagined the 8th Doctor in his Time War/ Dark Eyes Gear.
I somehow dislike his TV Movie outfit, so even the pictures of him on Chimes of Midnight or Human Resources or Seasons of Fear (one of my favorites), I only ever imagined him without the long hair and without that velvet jacket. I also imagined him as the older McGann.
When I finally got around to watching the TV movie, I was a bit surprised how young he looked there, since I had always imagined him older.
I still did not like that costume.
The TV movie also was better than what I had imagined from what people said about it (somehow I sense a common theme here, I should stop listening to what people say and make up my own mind!).
However, the naive, slightly loopy, enthusiastic and childlike Doctor of the movie was a bit alien to me after having listened to all his later Dark Eyes/ Doom Coalition/ Time War stuff. For me, there was a real glaring difference.
To this day, I prefer the darker, grumpier and sarcastic 8th Doctor to the early days one.
Caerdroia was therefore another eye-opener for me ( it is amazing how McGann manages to make each of the 3 Doctors distinct and you can tell them apart by just a single word said) and for some reason, the grumpy nasty version was my absolute favorite there.