Subwoofer connection to older tube amp

As explained to me by the main tech at HSU, the builder of my sub, the second set of speaker cables going to your sub from the main speaker terminals aren't drawing any appreciable power, they're just picking up frequency and amplitude information to be amplified by the amp in the sub. He said the gauge of the cables going to the sub doesn't even need to be the same as your main speaker cables since they're carrying very little power.

Thanks Sfar. If I can just run parallel wires off the same amp out connector that sounds easiest.

I appreciate Steuspeeds directions for using the line level outputs but as long as there isn't a real power drain on the D-90 and the sub amp is being used I'll do the joint speaker wire set-up.

That is how the high level inputs on a car stereo amp work for head units with no pre-amp out. It should work ok then. The down side is that the signal is amplified by the D90, un-amplified and re-amplified by the sub amp. This is likley to produce muddy bass. For best fidelity use direct rca line level connections with splitters like the Audioquest.

OK, but I'm still not sure what line level connections I have on the pre or the amp. The pre may have those but the amp really just has screw posts for speaker outs at different ohm levels. The only RCA style inputs is the Main for the pre input.

The sub has RCA line levels connections but not sure about these older tube electronics.

Sorry if this explanation is simplistic but I'm not sure where your confusion is.

Your power amp has two kinds of connections, both of them inputs. The first is the right and left inputs coming from the outputs of the preamp. Those are the RCA connectors on the back of the amp connected to RCA connectors on the back of the preamp. Those are line level - output from the preamp to input on the amp. There aren't any line-level outputs on the amp, only line-level inputs. The only output from the power amp is high-level, also called speaker-level.

The second kind of connectors on the amp are the speaker posts. Those are the speaker-level outputs that go - obviously - to your speakers.

The preamp can have a couple of kinds of outputs, the one kind every preamp has is a pair of line-level outputs controlled by the volume knob, like the ones going to your amp. You might also have another pair that aren't controlled by the volume knob, intended to go to a recording device and usually labeled "Tape out" or something like that. That's no use to you in this situation, even if you have them, since you want the signal going to the subwoofer to be controlled by the volume knob on the preamp.

So, you have two choices. First option is to use speaker cable to connect between the speaker terminals on your amp and the speaker-level inputs on the sub.

Second option is to split the output from the RCA connector outputs on the preamp using a 'splitter' as Steuspeed suggests and use two interconnects, one pair going from the splitter to the RCA inputs on the amp and the other pair going from the splitter to the line-level inputs on the subwoofer.

Some preamps have two sets of line-level outputs on the preamp but most don't. I'm assuming here that yours is one that doesn't.

The option I was recommending was option 1, the option Steuspeed was recommending was option 2. Either of them will work, which works better for you depends on your gear and what sounds best to you.

Sorry if this confuses things further. If it still doesn't make sense, please describe in some detail the connections you have on the back of your preamp.