[Gvt1911]

Friend got a Mirage powered subwoofer for cheap.   Only has one RCA input and also has R/L speaker inputs.  No outputs of any kind. His tube preamp has 2 outputs. One is being used for the main power amp. He was advised NOT to use a "Y" connector and that using the speaker inputs on the sub is not recommended. Any input on how to connect  ???

[JRace]

Who said not to use a Y adapter?

If you really do not want to then just use a mono cable on the LT or RT outputs on the pre.
There is very little (if any) bass that is in stereo.

Most often a Y adapter is used.

[JohnR]

You don't want to use a Y connector from the preamp outputs to the single sub input, as that will short the left and right together and then you'll have mono (at best).

The best bet would be to connect the outputs from the power amp to the speaker-level inputs. That's what they're designed for. This typically requires that the amp have the "negative" connection be a ground. Some amps like class D amps don't meet this criterion.

Otherwise, he could buy a cheap mixer from a music store to sum the left and right signals from the pre and feed the sub input with the output from that. Or just connect one channel as suggested and see how that goes...

[Speedskater]

Not that I haven't "Wyed" a million times (well, maybe a dozen) but this Rane paper points out that it's a bad idea. The output of one channel is trying to drive the output of the other channel.

http://www.rane.com/note109.html