Stereo sub out to mono, is it safe?

Al...no worries...this is audio and there are more ways to skin a cat...and I believe we all have good intentions...I just try to inform others of thje pitfalls I have encountered...this situation isn't one of those...99 percent of the time it will work...happy listening....

Don't you need a 'summing' adapter for this application?

I don't think this is the same as 'splitting' a single output which a vanilla flavored "Y" adapter would do.

But 'summing' two signals into one would take a different "Y" configuration.

The quickest example I could find was the one on Antimodes website:
http://www.dspeaker.com/en/support/using-stereo-pre-out-with-8033.shtml

Thank you Al. Appreciate your comment. Phasecorrect, I respectfully suggest you read my threads on the subject. Tom Tutay's buffer solved several problems: (1) asymetrical loading (i.e., output to amp was XLR; output to Paradigm control box or the sub was SE); (2) combined output impedance presented to my linestage was lower than factory recommended, the buffer increased impedance; and (3) the device summed L/R channels without shorting the linestage.

Too technical....I like music...all the best

Erikt raises a good point about the possibility of using an adapter containing a pair of resistors to do the summing. That can be thought of as a passive mixer. It would sometimes, but not always, be a viable (and less expensive) alternative to an active mixer in situations where directly shorting the two outputs together with a y-adapter would not work well.

It would eliminate the possibility of the particular distortion effect that I had mentioned, as well as whatever slight possibility of eventual damage might exist with the y-adapter approach. However, if the sub outputs of the amp are not separately buffered from the main signal path, while a mono signal would not be heard from the main speakers there would probably be a significant reduction in channel separation. A properly chosen active mixer would eliminate that possibility. Also, the high impedance of the summing resistors may or may not be sonically optimal for driving the sub, as powered subs often have relatively low input impedances compared to the line-level inputs of other components.

Regards,
-- Al