Adding a sub woofer?


Running a Planar 6 to Icon Audio Ps1 Mk2 to Quicksilver Mid Monos into Klipsch RP8000F speakers. Want more low end, thinking of adding a sub. The mono blocks don't have a dedicated sub output so I was thinking about something like a PSB 250 which has both pre amp and speaker level inputs with speaker level outputs. A couple of questions.

What are the pros and cons of using the sub's preamp level inputs vs the speaker level inputs in this application?

And more importantly, right now my setup is pretty much tube analogue the whole way (and aside from the soft low end, I absolutely love the overall sound) - but does an inline sub like this do any processing, will it color the mids and highs in some way or is it just a clean pass-through in terms of sending the signal along to the speakers? 

Thanks for sharing your thoughts. 
Ryan
spotconlon
Do you find that once you get the crossover and volume set for your system on a traditional single box sub that it doesn’t require adjustments?
I might have a situation similar to yours. My speakers go to 20Hz but I have a standing wave in the room that cancels bass at the listening chair even though elsewhere in my house the bass threatens the plaster. To break up the standing wave I added a pair of Swarm subwoofers, asymmetrically placed in the room. Once I got everything working I did not have to make any further adjustments regardless of the album playing. With a single sub this would be harder to do since you have less ability to break up standing waves. So how much bass you get will vary from album to album.
Regardless of Millercarbon's snarky condescension (per usual), RELs are NOT inherently "lumpy" unless set up badly, and a single one can work. Not everybody has room for a Swarm of four subs and many can get along with one well placed REL or another well designed brand. And there is a big difference between high level power amp driven signal and a single ended preamp signal as ideally the sub should sound like the amp, and the high level REL style works swimmingly. For reasons noted by atmasphere and others, using two subs (I use 2 RELs) can eliminate standing wave issues and get a room in tune easily, but if you can't afford 2 subs, simply get one and add another later if you feel the need.
One subwoofer can be very frustrating to an astute listener. Best to get at least two to start. Then they are matching. If you can't afford two save longer. 
If you use an appropriate crossover they will not color the midrange at all but interestingly give you more sense of depth in the midrange. There is more air in the room. It is an effect that most people notice. Turn off the subs and everything goes flat. But, the crossover is critical and the only ones that offer total control of frequency and slope are digital. People who are married to total analog miss out on this. JL Audio makes a decent analog crossover. 
I'm an "astute listener" and years ago added a REL Q150e which transformed my system...the adjustments on that REL are comprehensive for phase, level matching, and "crossover" matching, all easily done for excellent sound. Digital woofer control can add delay that isn't so great so I avoid digital nannies.
I don't know that I have ever appreciated a change in the midrange of my system with the subs active.  It is very intriguing how this could even happen with nothing changing at the signal the main speakers are receiving.  I suppose it is all about room acoustics and not what is leaving the speakers' cones, but what ends up getting to your ears.