The A1000 is not worth the extra. Not for the cosmetics (too similar to the A900 in all tactile respects), not for the sound. Amping? It's not really necessary with the A900 and extra power won't really affect what the phones can do... although of course different amps have different sonic influences. I would say if you plan to amp in a decent way, upgrade sources, etc anytime soon then you're better off considering another more capable phone. If not, the A900 is a keeper as IMO it's an excellent all-rounder with any budget-level source.
If you want something that improves a lot with good amping and from a precision source and it HAS to be a closed A-T, then the W1000 would be the best place to start. It takes to tubes pretty well, although the low ends are somewhat thin. If the Audiovalve RKV and the like are in your amp future, then the W1000 is not a bad choice (although I think you can make as good or better choices from other manufacturers).
The CD3K's can compete on some levels with the W2002. But it can't hold a candle up to the W2002 for precision of delivery and capturing the nuances of a great performance. However this is with the caveat that your 'back end' must be capable of generating that extra precision and nuance to begin with.
On another related note, if you find that you can tell a 'not worth the difference' minimal difference between a CD3K and an A900 when amped, then it's time to think about a new source/amp. The two are very different in capability, but this will only be exposed if your 'back end' is up to producing the differences in the first place. I think that may be lost on quite a lot of people here.
