How to split pre out to connect power amp and sub

There is a more significant issue to what you are proposing to do, which is that a tube amp should NEVER be operated without a speaker load, at least while music is being played through it, or it is likely to suffer serious damage (such as a blown output transformer).

By running the Shanling's pre-outs into the Prima Luna power amp, with the speakers connected to the Prima Luna power amp, that is exactly what would happen to the Shanling. If you would like further technical explanation of that, let us know and I'll answer tomorrow if necessary -- I have to sign off shortly for the night.

Apart from that, the answer to your question is that you could split the pre-out's with a y-adapter and send those outputs into both the sub and the power amp, provided that the parallel combination of the input impedances of the sub and the power amp is substantially higher than the output impedance of the preamp outputs. An example of "substantially" would be a 200 ohm preamp output impedance working into a combined sub+power amp input impedance of say 10K (10,000 ohms) or more.

If the impedances don't meet those criteria, no damage will result (as far as that issue is concerned) but sound quality may suffer.

To calculate the combined input impedance of the sub and the power amp, divide the product of the two impedances by their sum. Example: If the sub's input impedance is 25,000 ohms (25K), and the power amp's input impedance is 50,000 ohms (50K), their parallel combination would be (25 x 50)/(25 + 50) = 16.67K.

But again, DON'T DO IT, or you will destroy the power amp section of the Shanling.

Regards,
-- Al

P.S. The comments in my previous post about the y-adapter and impedance matching are based on the assumption that your sub has separate left and right channel inputs, or alternatively that you have a separate sub for each channel. Upon re-reading your post I'm not sure that is the case -- please clarify and I'll answer further tomorrow, if necessary.

Regards,
-- Al

A further thought: Since the Shanling has such a low power output, that would make it readily practicable to substitute a resistor for the speaker load which would no longer be connected to it. Doing so would completely eliminate the possibility of damage that I described!

Radio Shack part number 271-120, which is an 8 ohm 20 watt non-inductive resistor, costing $1.49 each according to an older catalog I have, would do the trick. Get two of these, and connect one between the Shanling's red and black left channel speaker terminals, and connect the other one between the red and black right channel speaker terminals.

Hope that helps,
-- Al