I know what you mean. Settling on one amp possibly narrows down the list of suitable headphones for your tastes. However that might not necessarily result in the best sound quality or value for money just simplicity (or lack of choice depending on how you want to look at it).
I thought headphones were more important for SQ than amps? If that is the case it might be better to decide on a headphone and then find out from current owners which amps go well with it.
I used to be in awe of the O2 till I actually owned/heard it, now I'm a believer of systems and synergy.