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
After moving some years ago, my listening space became smaller and the Vandersteen 2CEs I had were no longer a good fit. Replaced them with Kef LS50s and enjoyed the change but missed the low-end of the Vandies. Finally bit the bullet and got a Rel T/5i sub to provide the missing bass.

Using speaker-level connection and corner placement, as recommended by Rel, I dialed everything in using 1/3-octave warble test tones and SPL meter.

Adding the sub noticeably increased the soundstage (width & depth) and made for a much more enjoyable listening experience. Highly recommended for anyone with small main speakers. 
Hi I agree with Hshifi, front firing subs 
are good for listening to music down firing 
subs are good for home theater .
and two is better than one if your room 
can accommodate two and also cost.
i have two Rels S/510
Happy listening.
Fi.

I have 2 REL S5 SHO's on Pass Mono Amps.  I am using the REL LongBow wireless system.  It allows me to only worry about providing power to the subs not running wires across the room.  I have one near a far corner from the speakers and one behind me near another corner.  The Longbow connects to the Pass via the speaker outputs via an Analysis Plus cable made for the REL's.  It gives extremely fast and articulate bass not boomy.  The advantage of using the amp output is the subs see the same signal as your speakers.  Using a good digital power cord improves the imaging.  I replaced the smps with iFi power supplies.  Adding a PS Audio Noise Harvester to the outlets also makes for better imaging.
By what metric do you ascribe orientation preference?

IMO in the last 20 years, REL has degenerated into BuzzWord Bingo marketing hype preying on the uniformed:

" PerfectFilter performs two seemingly disparate qualities; extending the strong, even response of the extreme low end of bass frequencies while simultaneously opening up air and delicacy in the middle and high frequencies of one’s system. Additionally, we applied customized PureTheatre™ filters to allow S/510 to keep up with modern theatre effects without breaking a sweat."

Anything that changes the mid and upper frequencies is a TONE CONTROL!

30Hz is 30Hz REGARDLESS of whether it’s generated by Godzilla stomping or a JAV Recording https://pipe-organ-recordings.com/

Additionally, the S/510 is missing the controls necessary for seamless integration. Pass.
Having spent north of $50k on dedicated bass units over the years I wish I had this advice. There is a brand that is far and beyond the best I've owned and best of all, no where near the most expensive. Buy two SVS subs and be done with your bass search forever.