Losing volume when sub is hooked up

Do you mean the main speakers lose volume?
Does the sub have a line/spkr level switch?

Maybe it's in spkr and loading down the pre.

What sub is it? What integrated? You will get better advice if you provide details.

I found this spec sheet for the sub and it says the input impedance for the line input is only 2.5K. That is very low. That might be a misprint but it explains what you are hearing. I see other RELs that state 10K, also a relatively low value.

Your tube integrated probably has a relatively high output impedance from it's pre out. I couldn't find a spec for that. In any case the higher the input impedance is to the sub the easier it is for the pre out to drive it.

When you hook up to the line in on the sub you are putting the input to the sub in parallel with the input to the power amp section of the integrated. This lowers the impedance that the pre sees and therefore the output voltage drops and therefore the volume goes down. You really aren't losing any power as long as you can turn it up a little bit to get the voltage back to where it was.

Your real concern is that as the impedance drops it will tend to roll of the bottom end. However, since your are adding a sub you can probably balance things out by adjusting the sub. I really don't see it as a problem but if it bothers you you could always just use the speaker inputs to the sub. Another choice is to raise the input impedance of the sub. It is probably set by a single resistor at the input to each channel and a competent tech could replace it with a higher value. I did this with an MBL sub I used to have and that worked out just fine.

Yes, 100K input on the speaker/high level inputs is a very easy load for your integrated. I agree that is the best way to configure the sub in your system.