Running my KEF R3 with a Yamaha A-S701 and find them a bit dull-sounding, particularly in the lower frequencies. Would a better amp make a difference or replace them with something warmer?
A better Amp is unlikely to make a difference.
The R3's bass response is shelved down to mitigate room modes, but depending on the room this can backfire, resulting in weak bass response.
The first thing I'd try is to apply
@pierre's anechoic EQ correction for the R3, which partially removes this bass shelf (attached below).
If that's not enough, then it'd be time to measure your in-room response using a UMIK-1 measurements microphone, or in a pinch, the free HouseCurve App for iOS.