This Kustom PA KPC12P Powered Speaker Pair has that 70 Hz and 20 kHz specs but it's only 100 watt RMS!
Depends on the design of the box! You can't really compare a cab that was purpos built for a bass guitar and a FR cab. A good FRFR cab will usually have a response of +/- 3 db between 70 Hz and 20kHz @ -10db.
Most normal people don't hear past 16 kHz is and most guitar freqs don't have any energy past 10 kHz. Bass guitar is the opposite direction depending on how crisp you want you bass tones to sound. As to how the Head Rush will sound with Bass guitar running through it best way to find out is to go demo one or the Alto TS212 as it looks like the same speaker without all the marketing BS.