Mark,

One problem with your approach is that the volume control does not affect the tape out jacks so that level matching would only be correct for one setting of the volume control. You could split the preamp outputs, sending one run to the amplifier and one to each subwoofer. You would not be able to use the bass management in the RR2150 but could setup the crossover on the sub to match the natural rolloff of the speakers as closely as possible. In addition, you can use the volume pots on the subs to do level matching.
This would allow true stereo subwoofers without purchasing any additional electronics.
However, since most deep bass appears in both channels, I'd still prefer to run the mono sub out signal to the pair of subs (as I have done in my system).