James63,
75dB is definitely lower than I listen, and I've got the system balance optimized for my (higher) preferred levels. Our ears' frequency response sensitivity changes with SPL (e.g. Fletcher-Munson curves), so I would definitely set the system up differently. That said, the Kensingtons are so responsive to upstream gear (e.g. tube changes) that this should be achievable.
My current system is built with an emphasis on a warm, sweet and prominent midrange, with smooth top end (that won't kill my ears) and strong bass. For lower SPL levels, I'd want to focus relatively more on treble & bass. At my levels, the bass feels deliciusly plentiful and powerful, with no artificial boost necessary. In other words, there isn't any 1 system configuration for any 1 speaker that will be optimal at different volume levels. That said, the Kensingtons are capable of superb bass reponse - and here you can truly say "excellent bass reponse" without the usual qualifiers: "for a small-ish floorstander" or "for a monitor". Anytime I see a qualifier like that, no matter how sugar-coated, I stop reading further! You need serious size for real bass quality & quantity; the Kensingtons are the entry level to "big boy" sizes, and Tannoy knows how to use it.
Going a step further - I know how you feel when you look at the Glenair 15" and start thinking it must have even better bass response. I've heard them on my own amp & preamp (though only for a couple of hours), and though awesome, they didn't fully blow me away. The slightly sweet midrange of the Kensington wins it out for me. I didn't hear it give significantly better bass response either; it may be (wild guess here) that more of that extra size went to efficicency rather than bass extension. I should also note that both the Glenair 10" and Kensingtons seriously hopped-up their bass response when driven by a super-powerful tube amp (Rogue Apollos). Also, a couple of my friends heard the same setup and feel that the midrange on BOTH the Glenair 10" and Kensingtons are superior to the Glenair 15". I personally didn't hear any anomolies that could be attributed to the crossover; I just liked the Kensigtons better. Didn't do quite enough comparison to formulate my own midrange opinion on Glenair 10" vs. 15"; but my feeling is that it's close.
I've also heard the Churchill Widebands in a fully different system, and now THAT'S where the Tannoy 15" shows superior bass response! Seriously, seriously wicked dynamics in that system - best I've ever heard. Clean and neutral overall tonality. But again, the Kensingtons have a touch of midrange sweetness that I'm just drawn to - like an RCA black plate 12AX7/12BH7 or a Koetsu Platinum. When I looked at the Yorkminster & Canterbury I wondered whether they would have the Kensington's mids PLUS the Churchill's bass and dynamics. However, my friend with the Churchills has heard the Yorkminsters, and says that the mids on the Kensintons are just unique - in fact he said his Tannoy contact said they've got the best mids in the Prestige line. This friend also recently bough the Kingdom Royals, which I'm going to have to hear soon :P
