Those 15 inch woofers can sound slow and mushy with tubes.
The size of the woofers has nothing to do with the speed of the amp; this is a common amplifier myth. It turns out that if the woofers are over-damped, that that can slow them down, IOW if you have excessive damping factor the woofers can be slowed down, and the result is decreased output as I mentioned above.
Here is a reference article about this issue:
"Critical Damping: Missing Link in Speaker Operation Parts 1 & 2
http://www.pearl-hifi.com/06_Lit_Archive/07_Misc_Downloads/Misc_Downloads.html