Does the REL have a line level input - you could simply feed it a signal from your pre-amp?
You should be able to feed the two subs the same signal with a Y connector. This will lower the load seen by your pre-amp but not by enough to matter. If you don't like the idea of a Y then you may be able to use a Tape Monitoring/Recording line level out from your preamp and feed one sub the left channel and the other sub the right channel.
You should be able to feed the two subs the same signal with a Y connector. This will lower the load seen by your pre-amp but not by enough to matter. If you don't like the idea of a Y then you may be able to use a Tape Monitoring/Recording line level out from your preamp and feed one sub the left channel and the other sub the right channel.