Subwoofer connection to a tube preamp

"My sub has line-in (R&L) and line-out. However, the line out has a high pass filter which cuts-off at 80 HZ. "

That's what it is supposed to do: cut off the lows from the main speakers so that only the sub is reproducing the bottom octaves. How well it works depends on the quality of the unnamed sub, of course.

Kal

If it is to be used as a 2 Channel supplement, then you will get more musical results simply running speaker runs of wire to the high ins from your main amp... But do not use the loop back out to the speakers just keep those hooked direct to the amp as well, so use a seperate speaker cable set... Beyond that the RCA spliter could be used but you cannot go into the LFE input on the Sub because it will bypass the sub's plate amp crossover and will play full range, in other words not only play 80 hz and under, so if you use speaker wire to hook up the hi-lev inputs then you can set your crossover at 40 hz or possibly 60 hz for those speakers, or the standard 80 hz if that sounds best, but I know lascala's would be best off supplemented probably crossing over at 100 hz on the plate amp, but this will all be decided on what sounds best in your room. If you look at the Best high efficiency speaker systems that are Active bass modular, such as Avantgarde Duo's or Zu Definitions they are very simply run Line level so that the amps powering the LF drivers are seeing the exact flavor and signal of the amp feeding the mid and tweet's... Now if you want more Boom than using the LFE with a crossed over processor will provide 6db more input and output than running it line level.

Y ou say that your pre has no sub out, but does it have a line out other than the main line out? If yes, I'd use that. If no, simply split each channel of the main line out w/a Y splitter. That way you'll have a stereo input at your sub. I've done this and it works fine.