Try it and see if you haven't done so. 
I wish I could, Henry. The output of my First Watt F1J floats DC, so using its speaker outs is out. I actually forgot once, many years ago, and hooked the Dayton sub amp I was using at the time, up to the F1J outputs. A little plume of white smoke out of the Dayton quickly reminded me. Luckily I caught it in time. A resistor in the Dayton amp got hot, but no permanent damage done. Ed later reminded me on the old forum, "You can't
do that!"
When I use any of my tube amps, I'm able to follow your advice, and run the output of the tube amp into the Crown XLS 2002 RCA inputs, and it works great.
I've tried using a Y adapter on my Truth's RCA outs to run the First Watt and Crown in parallel. That creates a 5K ohm load for the Truth to drive, which should be no problem for it. But I seem to remember some sort of out of phase issue occurring (using a Stereophile test CD). It may have something to do with the fact that I'm running the Crown in bridged mono, but I'm not sure, and Crown never replied to my email. Ed said to call him, and I did, but we didn't connect for some reason. I went back to using my First Watt without a sub, but luckily the bass is so solid with that amp that I don't miss the subwoofer.
Thanks for the reminder, because I have all the "fixins" for a pair of Ed's bucket subs here, and need to talk to Ed to see what his suggestions are.