What is your system by the way????
Nothing fancy, but I like it. I've got a pair of Spendor SP1/2Es driven by an Image 65i tube amp. Source is a Squeezebox through a Lavry DA-10 DAC.
Spendor makes their own mid/bass units in house and use a fairly high crossover point to the tweeter. There would be no practical substitute for the mid-woofer and they are already using Scanspeak for the tweeters. Use any other drivers in that speaker and you've completely defeated the purpose of buying a Spendor. You're in the same boat with any high quality design.