Wow, I'm switching back and forth between my HD25's and SRH440's right now, and the 440's make the 25's sound downright cold and dry. Very odd. I never thought of the 440's as warm but with the bass boost turned on on the C421-OPA2227 they are actually pretty good sounding. Also very light and comfortable on the head.
Now on the other hand, these things have pretty poor synergy with the iBasso D-Zero. The 440's definitely need a bass boost, since they are pretty anemic down low without it. With the D-Zero lacking a bass boost, the low end is lacking pretty badly. Also, as I experienced with this amp and the HD25's, the D-zero makes them sound congested in the mids, lacking in bass, and rolled off in the high-end. Definitely not a recommended pairing. The D-Zero sounds great with the M-Audio Q40's though.
Okay so back to your query--yes, the 440's sound better amp'd, if you pick the right amp. They seem to sound quite good with my JDS Labs C421 with OPA2227 op-amp. They also sound very good out of my home theatre receiver. I'd never use them with the iBasso D-Zero. And that's all I have around to test with. I'd rather listen to them right out of the iPod than use the D-Zero, but I'd pick the C421 over either of those options any day of the week. Even more so with the bass boost on.
Hope that helps!