I would propose you start with replacing the speekers to get the best out of the head unit and not the other way around.
Unfortunately i have just got rid of my GTV. But i was using the original TAPE, yes TAPE head unit but had upgraded the speakers and the sound was excellent. The tapes i used were created on my home system on a Nakamichi tape deck which made all the difference. But simple upgrade of good quality mids and tweeters, sensibly and solidly mounted, was in my mind a better option for me than spending hundreds on a head unit and still having crap speakers. Plus the dash looks far better with the standard unit. There is no point in having a good source if you are listening to it through cheap paper cones! I had Infinity 61/2" mids and tweeters MDF mounted and secured on an adapted rear shelf and Alpine 5" mids and tweeters in the doors again MDF mounted.
Any way, hope you achieve the sound quality you are after which ever way you go.
a non Alfa owner.