Does anyone already have a sub in their valve system and found it to be an improvement ?

I am toying with the idea of trying one but;

a) Not sure if it would add anything to the overall sound.
b) Would it be compatible with a Leak Stereo 20 and Croft Pre-amp.

My reason for trying this is quite complicated and I won't go into everything (it'll halt you in your tracks due to boredom) but in simple terms I have a decent pair of B&W DM2 speakers and want more bass.

I'd love to try some decent Tannoys but budget dictates otherwise.

The DM2 have just been in for an inspection/service. The upshot is that, for my hearing, the higher/mid frequencies are quite forward and mask the bass. Others find the overall sound fine. A sub woofer was suggested as a worthwhile experiment to see if that would produce some of the lower frequencies I am struggling to 'get'.

From a cursory search on-line all that I find are references to modern day amps being connected to subs and/or that the main speakers cross-overs are over-ridden by the circuitry in the sub. Of course this is just my interpretation of what I've read and may not necessarily be correct.

Therefore hoping to pick your collective brains to see what has already been done (or not).

Cheers.