hi paul, most tube amps i have used seem to me to have too tubby bass when driving the big harbeths, i would say only audio research ref series tube amps (i have ref75 and ref110) approach the neighborhood of acceptability in that aspect... of course you do get a slightly sweeter treble and even more embodied voices etc from those tube amps, but i personally feel even with very good solid state (hegel, pass, ayre, primare, exposure, agd ... ) the mon 40 does voices and natural tone incredibly well already without tubes, and the hegel or similar ss amp driving the speaker preserves the midrange goodness while providing an enhanced sense of overall transparency, speed and transient clarity in the midbass down
as for the hegels, the h390 has a subtly softer touch to its treble, thus is a little more tube like (they voiced that model that way on purpose), while the 190 and 590 are very neutral/extended in the treble, airy, great sense of straight wire w gain, iron fist/velvet glove...