Your poweramp is rated at massive 400W right?
if I read the link you sent correctly , that sex amp is a headphone amp primarily.
I don't know, I am out of my depth about what speakers and configuration you are using.
But again, generally 450HZ upwards, is most of the audible bandwidth, even with a 110dB sensitivity unit, I think you need a topnotch, extremely quiet (hiss wise) amp with a few watts atleast.
Just because the unit is rated 16 Ohms, it does not mean it is that all the way through the bandwidth.
You should not be scared of OTL amps, I have used one for over a year now, but if you only need a few watts, go for a single ended (triode) amp.
What's your budget?
A single ended EL84 based amp should give about 3 to 5 watts of clean output.
Since your bass amp is a module, are you sure you want a tube amp?
Pickup a quality transitor amp from 70's or 80's from ebay, if they do not output bass, they do a very good job of higher frequencies.
Meridian 103's or sugden A21's in good condition would be fine.