Best way to hook up REL sub with single RCA input to preamp/amp combo?

One way I like to connect my speakers is as follows:

Tube preamp (Quicksilver) --> Integrated amp (Atoll INT200, using the "bypass" on the amp so I just use the amp portion only).

The effect is nice -- a hybrid setup. Solid state power with tube preamp qualities.

However, I also want to connect my single REL sub using an RCA interconnect. (I have the high level wires hooked up to my QS tube amp and don’t want to disconnect them. Too inconvenient.)

The question becomes, How do I hook up my REL sub's single, line-level RCA input to the other gear?

I tried two ways:

1. QS preamp --> REL. My QS preamp was specially set up to have two preamp outputs. In this setup, one preamp out goes to the Integrated amp section.

Y-splitter method. Problem. But when I sent the second preamp output of the QS to the sub and used a Y-splitter, the soundstage collapsed.

Single RCA method. Better but not stereo signal. I sent the second preamp output of the QS to the sub WITHOUT the Y-splitter, it was ok (but now I’m only sending either Left or Right to the sub. Seems less than optimal.)

2. The second way to connect the REL sub was to the pre-out on the Integrated. That seemed to work ok.

So, if I’ve described this adequately, how would you hook my single REL sub? To the tube pre-amp? To the solid state integrated?

Please don’t suggest a second sub. I’m already looking into that!