Setting up dual subwoofers to a two channel amp.

This is all great info. Thanks so much!

I had a 2.2 set up with a single driver speaker, matching subs, and a 300b set amp. I ran direct from the amp to the main speaker then a jumper from the main speaker to the powered subs. I tried to dial in the subs as best as possible, but had nulls and phase issues at the crossover point that Jjrenman referred to.
I have since gotten away from horns and have a 2.2 set up using monitors and matching subs with a Dspeaker Antimode Dual Core 2.0. In reality you have an active system with each channel having it's own dedicated amp. Which will only need to provide enough power to run the driver to full excursion. Very smart for good sound!
I use my Dual Core as a preamp and dac with a transport to the internal dac. There are 2 digital inputs, optical SPDIF and USB, XLR and RCA analog inputs, with XLR and RCA outs.

So how will this work for you?

With the Dual Core you can measure the room and see where your horns begin to drop off and set your subs crossovers accordingly.
ie: My monitors drop off steeply at 100hz. I use a XLR to RCA cable from the XLR out to a single ended amp for the monitors. Then from the RCA outs to the subs amps dialed in at 100hz. I Then run a room correction at 150hz (or which ever point you want to choose up to 500hz) which fixes room nulls, and phase issues, as well as correcting the crossover point. The Dual Core's algorithm is so good that if you miss the proper crossover point by a few hz, it will make adjustments to bring the frequencies to proper alignment.

The Dual Core are around $1100.00 and will give you the great sub to speaker integration, as well as tone controls, equalizer, room measuring, tilt and house controls, up to 4 different profiles to save corrections in, asynchronous USB input, optical digital out, and support fron Dspeaker for future firm ware updates.

Oh by the way the Dual Core is responsible for the best bass and over all sound I have ever had in my room. ALSO *I'm not affiliated with Dspeaker in any way other than a very pleased customer.*

Dspeaker Dual Core

http://www.simplifiaudio.com/online_store/dspeaker_store.html

Use an active pre-amp (SS is better than SET for bass response) with 2 RCA outputs to split the signal from the CD player/Media player/TT two ways - one direct to your 300B and then the Horns, 2nd using Y splitter further to drive each plate amp/subwoofer

My system is now PWD DAC II (has XLR and RCA both with active signal) - XLR to main preamp - 45 tube active and to my AN300 B monoblocks; RCA with splitter ie 2 lefts to left subwoofer bridged amp and 2 rights to right bridged subwoofer amp. Separate volume on PWD DAC, Tube Main Preamp, and adjustable X-over frequency and Volume on Subwoofer amps makes adjustment and blending seamless.

An interesting solution kelvin. I can do this with my Emm labs pre amp. Please forgive then ignorant question but I connect it to the volume out right? Rather than the line out? In order for the bass response to match the volume of the system as a whole?

Thanks for the advice, rodge but I think i am going to try to find as simple a solution as possible. As it is, my system is very simple and the components are as high quality I can get. Adding an additional component as complicated as the dual core you describe would not work for me. I am also using double backloaded horn full rangers and would prefer to stay with those and replacing my DAC/transport combination to anything other than the new playback designs stuff would be an automatic step back as what I have is the best I could find and afford.