Yes, Alot of Pre's have there own sound, which is why some people buy them.... I have a friend who swears by Joe Meek... Won't use anything else....
There are also Transparent Pres..... which are nice, cause you get to hear what the mic's really sound like....
I like having a few pre's with their own sound personally... When you get someone in the studio it's nice to have a choice of which one des the most justice on the vocalists voice....
Of course an outboard Pre will help your sound... Within reason... Like if you already have a mackie VLZ board and your thinking of spending $200 on an outboard pre, say like the tiny "ART" tube pre... I would say your better off sticking with your board...
But if your using a soundcard for pre's, your probably better off with anything else....
Im not sure about the plug ins.... If there are pre modelling plug ins.. I would be skeptical...... That would be like have a fiberglass Lamborghini on a Fiero Chassis rather than the Real Lamborghini.....
Bad analogy.. But, you know what I mean....
In your case, with a Tascam potastudio.. I would say YES, you are well served to go out and buy a pre in your price range.... I have one of them portastudio's for writing ideas down and the pres aren't very good...
Hope this helps.
Joe