Is there a reasonably inexpensive way to do this without investing hundreds of dollars in "audiophile jumpers" plus splitters?Yes there is. Fortunately the pre-outs of the Bryston integrated amps have low output impedances. For example, although that output impedance doesn't appear to be specified for the B60R, based on a look at its schematic at their website it appears to be in the vicinity of 75 to 100 ohms, DC coupled. That will minimize or eliminate the effects of the cable to the sub, and the sub's input impedance, on the signals that are received by the power amp section of the integrated amp. And of course the effects of that cable on mid and high frequencies that are received by the sub won't matter.
Also, assuming it is of reasonably decent quality the effects of the splitter itself won't matter, IMO. (I suspect that contrary opinions and experiences that may be reported are the result of cable effects and impedance interactions, and are not due to splitter effects in themselves).
I would suggest that you consider using the Audioquest hard y-adapter, and a set of good quality low priced RCA cables. Blue Jeans LC-1 would probably be a fine choice for the runs to both the sub and the power amp section of the integrated, although it is apparently not available in a length of less than 1 foot.
In saying all of this, I'm assuming that the sub has separate RCA inputs for each of the two channels. Post back if that is not the case.