Warm ..........they are biased fully into Class A for the first 34 watts so whatever you want to define 'warm as' then perhaps.
Having owned tube amps (Rogue M180's) and a Plinius SA-102 (SS Class A) I fully understand what your trying to say but as for 'bass slam' define that for me ?
As for the X260.8's, I own a pair driving my Revel Studio II's and they are super. They exhibit plenty of ability in both lower and upper octaves. Their superb damping factor combined with strong current / power capability is what determines bass delivering capability as I know it.
As for some reports that they get excessively hot.......utter nonsense.