Subwoofer high-level connection question

Shouldn't be an issue. I use the same hook up protocol with my ss amp. Both the speaker cables and the high level sub cable are connected to the same posts. You could check with REL, as their customer service is excellent, but i see no problem.

I suggest contacting the tube amp manufacturer as well. Essentially you are wiring in parallel the speaker load to the REL high level connection and connecting it to the amp. 

no problem with Pass or Belles...be aware that balanced amps are connected differently than single ended amps to the REL...can never hurt to talk to your amp manufacturer...

Is it any kind of issue if the spades from the main speakers are touching the bare wire for the REL at the speaker terminal they are both connected to?

Not a problem. And if you can spare a minute to learn a little electronics you will understand why its not a problem for any amp, tube or solid state, or any sub, or anything else for that matter. 

First, it can't be a problem because all you're doing is connecting another load in parallel. In terms of volts and amps this is no different than connecting another speaker, or changing from a 2-way to 3-way speaker. 

Second, its not a problem in terms of load. Your sub amp is designed to run on line level voltage not speaker level. Line level is around 2-3 volts. Speaker level, well its a lot more. So what happens, the first thing that happens inside the plate amp in the sub there's a resistor, probably about 20k Ohms but could be 50kOhms. That's a lot of resistance! This does two things. One, it drops the voltage the sub sees way down to line level. And two, it means the load the amp sees is essentially nil. There's essentially no current draw on this circuit. The amp never even knows its there. 

I know this is exactly how this works, by the way, because I used this to modify my tube integrated to provide a pre-out to drive my Dayton sub amps. There's even a picture right here on my System page. https://systems.audiogon.com/systems/8367  Those two resistors are all the difference between high level speaker out and line level pre- out.  

Third and finally, electrically, as far as the amp is concerned, this is the same whether the resistors are in the amp or in the sub. In the same way it makes no difference whether your wires touch on the speaker terminal or anywhere else. Its all the same.

Thanks all, especially @millercarbon for the detailed and useful explanation!  I had read that the sub has a 100,000 Ohm input impedance and that the amp doesn't really "see" it.  Your explanation is really helpful in understanding this phenomenon and I appreciate it.