Is 300VA-400VA per channel enough? Will less be detrimental to performance and SQ? Is more always better?
That's about 300-400 Watts per channel. Simultaneously. That's quite a bit.
What are ideal specs for a multichannel ( or stereo amplifier ) that will provide a pleasant experience. Not looking for the 'best' of the best... but for a decent amp... what are specs that I should be focused upon?
This is really dependent on the size of the room, the efficiency of your speakers and how close you sit to them. The efficiency of the speakers can vary by huge amounts. Get theater grade horns and 10-20 watts will be too much.
I think it's also important to note that 300-400 watts / channel is 1,500 - 1,600 simultaneously.
Specs never trump personal preference, but they do ensure that manufacturers don't try to cheat you too badly.