Bob_Reynolds 7-17-2016 5:16pm EDT
If your preamp has a 2nd pair of outputs, then use those for the sub. Otherwise, you can use a Y-connector to split the preamp output between the sub and main amps. The problem with using a y-connector is the effective impedance the preamp will see, which can be a real concern for tube preamps. The impedance of line level connections on subs are typically in the 10K ohm to 20K ohm range.
I would add to Bob’s characteristically excellent comments that in the majority of cases where preamps provide two outputs for each channel both of those outputs are driven by the same output stage circuitry, and are simply jumpered together just inside the rear panel. Which means that in those cases the impedance compatibility concern would occur no differently than if the preamp provided just one output for each channel and a splitter is used.
In this case it appears that your 16LS preamp does provide two sets of output jacks, and according to Stereophile’s measurements its output impedance rises to about 1800 ohms at 20 Hz, while being much lower at mid-range and treble frequencies. Assuming, as is likely, that both output jacks for each channel are driven from the same output stage, the output stage would see a load impedance corresponding to the parallel combination of the sub’s input impedance and the 98K input impedance of your amp. Which will result in an overall load impedance that is less than the individual input impedances of both the sub and the amp.
Therefore if you choose to connect the outputs of the preamp to a sub (or to two subs), as opposed to connecting the outputs of the power amps to sub(s) that can accept speaker level inputs, I would not choose a sub having an input impedance of less than about 20K. 20K in parallel with 98K results in an overall load impedance of (20 x 98)/(20 + 98) = 16.6K, which is already slightly in violation of the 10x rule of thumb guideline for assuring impedance compatibility, relative to the preamp’s 1800 ohm output impedance at 20 Hz.
Good luck. Regards,
-- Al