Dipole surround speaker to match KEF 104.2?


I have KEF 104.2 speakers for front L/R and a KEF Model 100 center channel speaker. Any suggestions for dipole surrounds up to $500 per pair new/used? Also, in a 7.1 system, should the "Surround Back" speakers be identical to the "Surround" speakers? Or can I use my old Paradigm Titans in "Back" with some new dipoles in the "Surround" position? Thank you.
javachip
Thank you for the good suggestions. The Dolby website does recommend identical speakers, but interestingly the THX website recommends "diffuse" surrounds (dipoles) on the sides, and monopole back surrounds placed together, for 7.1 use.

Maybe for critical SACD or DVD-A listening, identical monopole speakers would be best, but I plan to mainly use the multichannel mode for movies, and stick to 2 channel for music. (I may change my mind after I listen to a SACD or DVD-A, which I have not done yet.)

More comments/suggestions would be welcome.
I am using a $500 (new) pair of Definitive Technology BP-2X bipole rear speakers with KEF 101/2 for front L&R and the model 90 center. Sounds great even with SACD and DVD/A.
I use a pair of KEF 102/2s with my KEF 104/2s. They are located along the walls lateral to the listening position. The center channel is a KEF 200 C. I run all the speakers in bi-wire mode. Frequencies below 80 Hz are shunted to a Velodyne HGS-15. This arrangemnt is excellent for 5.1 Dolby Digital and dts. The 104/2s and HGS-15 combination is excellent for stereo, and blend seamlessly.

The KEF 200 C is a much better match for the 104/2s than a 100 C. I moved my 100 C downstairs to use with a second pair of 102/2s and a Velodyne HGS-10. The 102/2s and 100 C are well matched.

db
Thanks for the info. I will start looking around for a used 200 C. There are none listed on Audiogon or Ebay right now.
By the way, I think the 104/2s sound great by themselves, but relieving them of below 80 Hz duty with a sub really opens their sound. They are wonderful speakers, as are the smaller sounding 102/2s.

db