How to determine Preamp "Power"? VTL 5.5 vs BAT SE

I would like to get a tube pre and amp that has a low end on the heftier side. I think I understand that a tube amp with KT88s will have a heftier bottom end than that of an EL34 system (right?). So I have begun looking at a tube pre that will support the bottom end of a KT88 amp.
VTL has a reputation for a strong bottom end. The VTL 5.5 LS pre uses these tubes: 2 x 12AX7, 4 x 12AT7. The BAT 3iX SE (for example) uses 2 6H30 and 2 6V6 tubes. I have read that the 6H30 tubes will give more power at the frequency extremes, eg bottom end than EL34 and even KT88 (yes? no?). If so, how would the pre "power" of the VTL 5.5 compare to the BAT SE in terms of bottom end heft? Or, am i looking at this all wrong? (tube newbie). Thanks!