Small home amp: Ear Max Pro - lush, rich, musical
Home/Hybrid: Rudistor RP6-DQ or RP7 Lite - spacious, textured, excellent imaging, fast
Home/Tube: Audiovalve RKV MkII + Eddie Current EC/DC - the best I've heard so far for the DT880-2005 - very rich, big/deep bass, great sound space dimension (width/depth/height), great impact/weight, excellent imaging, delicious musicality
Small portable (A): "M" Hornet - liquid, excellent extension without HF fatigue, deep/solid bass, extremely wide and open soundstage for a portable (beyond the ears) with excellent air and depth. Delicious vocals and smooth musicality, "natural" timbral presentation, with great bass impact that does not impinge on the mids or highs. The "M" mod brings it closer to the SR-71 (not the same), but with more speed and punch - and of course the 3-gain switch, AC adaptor and internal charging (NiMH 9V only - no lithiums). Heavenly with DT880-250, K701 and ES2 (silent & sweet - best choice for ES2).
[EDIT/ADD:] Small portable (B): Portaphile PV2^2-LT1210 Maxxed - perfect if you prefer a more detailed & textured sound, this amp has the most articulated/defined sound of any portable I've heard (best-defined bass & percussion instruments -- with clearest notes), with superb timbral nuance, inner detail, well-defined shape and position of instruments/performers, and also a wide soundstage with great dimension. The Hornet has bigger and slightly deeper bass and lusher mids, but the extra HF extension of this one gives more complex layering instead. If you like guitars, this presents the rich bite and juicy "jangle" of stringed instruments. Internal DIP-switchs for hi/lo gain, Class-A bias, etc.; AC adaptor option, no internal charging; can take the iPower 500mAh Lithium-Polymer 9V rechargeable (requires separate external charger). A dramatic and exciting sound. Dramatic & Textured w/K701, wide & extended with ES2, turns HD650/Equinox into a clean/clear/fast/crisp&detailed phone.
Large portable: Headroom Micro with Desktop Module - big soundstage, well-integrated frequency spectrum, "large" sound, slightly-but-not-overwhelmingly warm character, excellent but not migraine-inducing detail, smooth and musical.