I would love to say buy Celestion, Fane or another UK supplier, but unless you have a wall of sound, how much low bass will they achieve? [ans: bugger all IME]. Volt seem way overpriced IMO and trade on self promotion, like ATC. I'm afraid I have to suggest Italian ie RCF

eg:

L15P200AK

 -

 15"

 800 Wrms

 95 dB

 35-2kHz

 Classic RCF Low freq sub woofer

 £146.00

 

from:

http://www.allangordon.co.uk/Drivers/Cone-Drive-Units.htm

This has a +-9mm linear throw which matches JBL stuff and is very long for proaudio drivers

I have heard RCF stuff sounding excellent, but you may want to confirm from others about ultimate reliability [year in year out wise] as I am out of that area nowadays