Lead surrounds?
I can't take any measurements at the moment, as the speakers are in the loft. I can listen up there, and measure impedance, etc. but not the response. And at the moment they are in pieces being fiddled with; new binding posts, new 4 pin chassis connectors and crossover components. When all that is done I can drag them down again (loft ladder and stairs to cope with) and get the mic out. Then they will have to be dragged back up, as the QUADs are still here. So I want to minimise all that, as I am not as fit (or young!) as I was, and I have to do it alone.
Changing VAS to 400 litres in WinISD, and playing around* with box, vent and tuning, I looked at a 150 litre box, 10cm vent and 23 or 26Hz tuning, to add thump, as you put it. My reasoning being that a larger box gives me more options, with cabinet wall shapes and driver placement height, it can be reduced in volume if necessary, the larger vent similarly (easy to sleeve down). Also Paul's dimensions are not far from that 150 litre size, so easier for me to scale my box, etc. At the moment I have domestic acceptance of those dimensions, and changing it too much means I have to go through all the explanations again!
I do have a pair of Ripole subs I made for the QUADs, so there is another option for bass reinforcement, but I'm not really expecting to need them.
* I say that as I am not sure I am getting it right. With a 150 litre box, one vent at 10cm , 26Hz tuning, I get -1dB@40Hz, -3@30Hz and -12@20Hz. At 23Hz, it's -2@40Hz, -4@30Hz and -10 or 11@20Hz. Vent lengths of 15.85 and 22.29cm. Does that match your numbers?