You would probably get some improvement but I can't say how much. If better amps are what you're looking for (and not additional features), I would recommend you spend your money on a quality separate amp and use your 2801 as a pre-amp.
I recently added an Acurus 100x3 to my 2801 and have been extremely pleased with the results. Not only do movies have more impact, soundstaging and imaging were improved greatly, as was clarity. The 100x3 is a three channel amp that drives the front three speakers. The internal amps on the 2801 are still pushing the surrounds.
There are a number of good, mid-fi, reasonably priced amps out there that might give you a better bang-for-buck improvement than going to the 3801.
Just a thought.
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--Elliott--