My favorite small amp is the Parasound Zamp - the v.1 and v.2 are 30W per channel and can be purchased used on eBay for around $100. The v.3 is 45W per channel and is very nice but these go for significantly more. Quite small - only 9-1/2"w x 2"h x 10"d
I use these amps to power my whole house audio system and have had very good results.
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2 / Paradigm Signature S4 v.2 (L/R), (2) SVS SB12-NSD (Subs)
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD