I'm not running 803's as surrounds but do use large towers comparable in size and low frequency spec as the 803 and absolutely love them. I use 802 Diamonds as my mains with a HTM2 Diamond center. Next month my 802's become my surrounds and 800's become my mains. The 802's will work wonderfully with the 803's in the surround position. thrang has 802's and the HTM2 up front and uses 803's as wides and side surrounds and he has no complaints. Kahuna Canuck has 800D's up front with 803D's as surrounds and is very happy with that combination as well.
Here are my feelings/observations on surround speakers. What you need to ask yourself is are you in the camp that feels surround speakers are not really important so it doesn't matter what is used (i.e. size, type, manufacturer, series regardless of what the mains are) or are you in the camp that either sees the importance of the surround speakers (or the potential of the speakers) or just likes to be enveloped in sound. If you are a music listener and enjoy using matrix modes like Dolby PLIIx, THX Ultra2, Logic 7, etc on 2 channel material to expand to a 5.1 or 7.1 or on 5.1 material to expand to 7.1 as this ensures the surround speakers get material on a much higher frequency then I have always felt the surround speakers become darn near as important as the front 3. If you don't use matrix modes to listen to stereo music, don't listen to multi-channel music, mainly use the sytem for movies and only engage the true discrete codes i.e. Dolby Digital, Dolby TrueHD, dts and dts-HD Master Audio and do not apply Dolby PLIIx, THX Ultra2, Logic 7 on top of those modes then the surrounds do not need to be a focus because 1, you are very rarely ever using the surround back channels (i.e. the 6th and 7th speakers in a 7.1 setup due to so few releases having 7.1 content) and 2, what is going to your surrounds will mostly be ambient effect sounds and it will often be dumbed down in volume anyway due to the mixing process. In case of #2 I would say either sell the 803's or perhaps use them as wides in a 9.1 setup if you want to experiment with that. Keep in mind though that 9.1 is of course a matrix mode so if you don't currently use PLIIx, THX Ultra2, Logic 7, etc then it's unlikely you would be using dts:Neo X or Audyssey DSX in order to activate the wides.
Wow, that was great info Rod#S, thank you! Yes, I love to be enveloped in sound and am experimenting more and more with matrix modes. My current use is more movies than music (unfortunately) due to my little one, but I plan to change those habits soon. I certainly agree that all speakers play an important role and see value in keeping the 803's around.