I always wonder why tube integrated manufacturers do not provide a preout for the use of a sub.I'd imagine that it is often because it would require an additional buffer stage (for each channel), to provide a reasonably low output impedance for the pre-outs, and to prevent the main signal path from being affected by what is connected to the pre-outs. Keep in mind that the impedance level at the output of a tube-based circuit stage is usually far higher than at the output of a solid state stage, resulting in greater sensitivity to loading.
That would generally require the addition of at least one dual-section tube and several passive components.
Regards,
-- Al