Power amp for Audio Research SP16L pre-amp

Depends on your speakers and other variables.. the VS55 is really a honey of an amp if you can deal with the lower watt output. The older VT series are expensive and more difficult to retube. The 300.2 is a digital amp and has it's own signature...

So it depends on how you are trying to make your system sound?

Well I will be using it with some Proac Reference 8 signatures.
The sound I'm after would probably be a neutral sound.

I would also consider the ARC 150.2 or the 100.2. Both have more than enough power for your application. I'm using a 150.2 with my SP16 currently and am very pleased. It runs cool, is so reliable and sounds great. I had a 100.2 which had a bit more texture and body in the midrange and slightly more natural highs. It is widely regarded as one of the best ss amps ARC ever built. It will cost you though--they come on the A'gon infrequently and sell quickly at asking price. For $500 less you can get a 150.2 that will last forever and sounds nearly as good.