Really, read Olsher's review of the Crescendo. The transmission line proveds clean, linear bass down to around 40 Hz. An important distinction of the Crescendo is its transmission line bass loading. Olsher makes a very strong point about how these speakers dramtically showed how he got used to the lagging bass of bass reflex designs, whereas the TL-loaded designs are quicker, more articulate, and especially rhythmic, i.e., more real sounding.
10-03-14: Dave_72
Those look good, Rhljazz, but do they have enough bass. It's hard to beat the 15s in the Tannoys and JBLs.
Bass reflex designs tend to have wild swings in impedance, something most tube amps don't like, complicated by the bass reflex's need for an amp with high damping factor, something a tube amp can't generally deliver. The Crescendo presents a benign load and presents no out-of-phase time lag. Win-win.
If you want that last octave of extension, it's far more economical to get a clean, articulate speaker down to 40Hz and then add in the last octave with a powered sub or two. True full-range speakers that don't let the bass extension smear the upper octaves rquire Herculean damping and cabinetry, making them crazy expensive--Wilson Maxx & Alexandria, Focal's top line Utopias, the big Magicos. These are $80K-200K.
Brand new and full retail Crescendos are $18K. A pair of JL Fathom F12s adds $7K. For $25K you have a true full-range system with sub-20Hz extension. Even a pair of their E110s would do the job for about $3K/pair, coming in at $21K and still tube-friendly.