Erik,
I have owned several prepros, several power amps, and several receivers used as prepros. IMO, the main advantage of separates over receivers is the amplication (especially for lower sensitivity, full-range speakers like your NHTs). Differences between prepros and the prepro section of...
Erik,
A Sig 2.1 amp direct from HK should be fine as a refurb, as it carries the original 2 year warranty (and $20 shipping is cheap). Other 5 channel amps under $1k like parasound 1205, marantz mm9000, etc should work fine too. However, the AC2 for ~ $500 plus a $500 amp IMO is better...
Another NHT/big amps fan here:
NHT 3.3, AC2, 1.5, one VS1.2a rear center
2 NHT SW3P subs
Onkyo 989
Two citation 5.1 amps bridged to 300x3 for front 3 and 100x2 for subs
You guys should both get the AC2 to go with your 2.5i mains, it will blend very well. HK amps sound great with NHTs, and...