65hz - 23kHz +/- 3db is the ligitimate spec here because -3dB is the half-power point. Output less than -3dB trends toward meaninglessness. The smallest change in SPL that you can recognize as a change in volume is +- 3dB.

54hz - 23kHz -10db is meaningless because -10dB is approaching inaudible and is certainly useless, i.e. -10dB will sound 1/2 as loud as 0dB and will be 1/10 the power as 0dB. Your ears are logarithmic.... remember log_10 from school?

What $ do you have toward making a change?

Any "sub" you might use would have to operate up to about 120 Hz, which is becoming directional, i.e. you can tell where the sound is coming from so you would need two. Also adding a "sub" can be complicated, so selecting speakers with a deeper bass response is probably less expensive and ln many ways your best bet. Lets say speakers with 8" to 12" diameter woofers.

Audio can be very much like fashion: last year's must-have hot item can have little value this year. Some people here do very well scarfing up excellent used equipment. Maybe you should pick their brains.