<<If you're looking for firm bass and added warmth....
Hang on.... I am not looking for more warmth and bass than I've currently got.... The Lavardin is a neutral amp but its on the warm side of neutral

, it manages to do natural warmth without any softness. I totally and increasingly understand that I may be foolish to think of replacing it.
Also, the Quads have plenty of bass.... there is a myth that says they don't but in my room which is not vast and with plenty of care over positioning I get full, rich and non-sludgy bass to a very good depth, for example can certainly can feel the organ pedal in Saturn from 'The Planets'.
The reason I asked about a valve amp with good tuneful bass is that my very limited experience of valve amps at dealer demos has left me with impression that bass was somewhat limited and slow... could be totally wrong here, and I cannot remember what valve amps I've heard.
What I really want to get is more presence, imaging, envelopment.... people who have heard my system think its got it in bucketfuls but I can never have enough.
Regarding subwoofers.... I have tried a subwoofer with very good results in previous incarnations of my system. Several years ago I had Ruark Templar sealed box mini floorstanders with a REL Strata sub - the combo was wonderful - not just the depth but the ease and scale of the music - and I don't think I had anything better than it until I got the Quads.
However, I have also tried using a sub with the Quads, just as an experiment, and it just doesn't work - integrating dipolar speakers and a sub is not easy, maybe would work with something like Tact RCS. I either got nothing from the sub or a horrible blanket of 'farty' bass.
I might think about the front-end again, but funds are limited as I don't have anything to sell, I'd definitely be holding on to the recently acquired Shanling for its superlative SACD playback. I'm pretty happy with its CD playback too.