Adelin, you have it right, and if these salesmen are telling you that all these quality amplifiers(including your 675)"sound different", they're either uninformed or lying. Amplifiers should only add more voltage from their power supply section to make the weak incoming voltage strong enough to drive speakers at a comfortably loud level. If they do this with audibly flat frequency response and inaudibly low noise and distortion, they've done all that's possible for a amplifier to do, regardless of the specific details of technology that were employed. Any amplifier that adds a "sound" is either incompetently designed or defective.
Your comment about your 675 isn't clear. If it's fine even when running the speakers full-range without bass management, it'd be even easier if a sub was taking part of the low-bass burden off. So, it would seem that you're okay and not just "for a while", and have no need to spend money on a separate amplifier.
