GeoffW, you didn't mention what amp you had. If you're using both phones unamped, the higher efficiency CD3000's will lead them to take top prize in the bass department. But by the same token, if you're not running the V6/7506 properly amped, you've nowhere near began to experience the tight and powerful bass that the V6/7506 is capable of. It's very impressive and a properly powered V6/7506 does considerably better than a correspondingly powered CD3000.
Radrd, I agree that some amp-source combos can work magic on the CD3000. Something with very high resolution and very tight bass will probably make them to very well. But it's relative, I think. If you can find a combo that does deep tight bass on the CD3000, it's my suspicion that sticking, say a A1000 on the same set-up will result in a correspondingly tighter and more powerful bass. And I did say comparatively flabby bass. The A-T's were/are very tight on my set-up. When I tried the CD3K with the META where all phones sounded significantly more bloated, comparatively it was as flabby to the A-T's as it was with the WAD amp. I have an even more bloaty SS amp and tried that out of curiousity... And once again, the degeneration was relatively similar.
Going to the other extreme, the replacement of the source with the XA777ES has tightened things up a lot from the previous CDP. And if I'd heard the CD3K alone in my current set-up, I might say "wow, that's really tight" but if I put on any of the A-T's after that, that impression would probably still be broken. I could be mistaken as I didn't have the opportunity to test the CD3K with the 777 but I think I have enough evidence to be sure.