Sure, tube amps typically don't have the extension (bass and treble) that solid state amps have. The CATs aren't any different in this respect and roll off deep bass and treble. Driving them with a preamp that is very extended in, say, the treble helps to make them sound less dark.
over the years I've had quite a number of tube amps in my room driven by the dart pre, both the original one, and the newest version. tubes like the darTZeel pre, as the dart does not add any grain, has extended silky highs, has outstanding micro dynamics, while lowering the overall noise floor and firming up the bass.
I used esoteric pre with big el34 tube amps with valve rectification and separate power supply for driver sections
The extremes were much better presented
The fact thus pre can also swing enormous voltages like valve pres a la AR 6 and 10 gives both synergy and great extension/ dynamics
It's the opposite of what's often said ie valve pre ss amps
However I found it a very natural sound
All instruments sound more true to life
El 34 is known for its midrange
The esoteric seems to add superb highs, and explosive bass to the mix ! YMMV