Which sub-help please

The Vandersteen 2WQ is what I think you are looking.
whether you are using lower powered Tube amps or any power amp the Vandy 2WQ and also the rel gets its input from speaker level.
This offers us better matching or blend from Main speakers with Sub because the main amps character is preserved.
Where the Vandersteen design takes this a step further is by a unique high pass which allows for optimized or best turnover point. This not only improves dynamics but dramatically lowers distortion increases clarity and transparency with the whole system.

(((If you want to really take the bass load off the main amp, then you have to look at bass management. I'm pretty sure the REL does not support BM internally. The SPL8R has a passive (I believe) first order high pass filter, but you'd probably prefer either an external BM controller or a sub that provides BM with higher order filters.)))
He is running a clean trube pre amp
What makes you think he would prefer a BM system which
would then have his pre amp going through a 50 Cent integrated circuit and lower input imp?
Cheers Johnnyr

(((You failed to mention that the Vandy's sub design essentially requires using two and that they neeed to be placed near the main speakers -- due to the first order low pass filter of the sub. Richard told me years ago to not bother buying a single 2Wq sub -- save up until I could buy a pair.))))

Bob This is True with any sub yes they are all better in pairs in a High performance Audio system.
The beauty with the Vandersteen 2WQ is they are flexible with set up and a more tranparent solution than anything thats been mentioned which again takes into consideration his tube pre amp by not loading it down IMP wise and BM ICs
or cheap OP amp wise instead using a more transparent time,phase correct simple High pass.
The 2WQ Subs are also designed to work in a corner if need be or wherever your favorite place may be because they have adjustable Q. by the way to the discerning ear all subs
sound best when near main speakers or near the same wall.
Cheers Johnnyr

Bob
If you like all that extra ringing of a 4th order filter system knock yourself out.
Personally I like to leave the thumpen and a bumpen to the
Cars that go by.
Best Johnnyr