Not necessarily lower .. my favourite Tannoy drivers are the 12in HPDs and 15in Golds .... the 12in HPDs need more grunt but in terms of specs go just as low as the 15in Golds ... in 12in Golds are very sweet but don't go as low as the 12in HPDs (not an issue in some rooms and most of the cabs they came in won't allow it anyway), while the ribbed cone on the 15in HPDs not only needs more power but isn't capable of the subtlety of the 15in Gold (or 12in HPD), and they don't really go any lower than either even in a big box.
All in all the 12in HPD is my preferred compromise between it and the 15in Gold (which I owned for about 5-6 years in multiple cabs including Lancasters, GRF rectangular horns, Lockwoods and Autographs) as while it doesn't shift as much air, it goes just as low (in the right cab) and is capable of slightly more more subtlety - perfect for my sort of music. The 15in Gold does give you a bit more of that giant room filling Tannoy sound but not quite the nuance I think mainly because it's pushed very high at the crossover point (that's a big cone to be doing 1khz - a couple of octaves above middle C) so slightly less clarity for the classical stuff - but potentially not so much of an issue depending on your musical choices.
In short Jerry's observations are probably spot on. I can see why the 15in Gold would be preferred by some, and I'm very happy with the 12in HPD.
Having said all that when Kedar came round I stuck a Sheffield Labs Wagner recording on (Siegfried Funeral Music) which has the most dynamic range I've ever heard on an LP, and we really rocked the place. Not something I do that often tbh.
For me the critical thing about Tannoys is to get the crossover right. When you describe mine as a one-off I think that's because so few people have really gone to town on the crossover - Paul did a fantastic job on mine indeed I would say it's the biggest single upgrade I've ever made to my system.