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
"I’ve heard that the interaction between multiple subs can cause problems"

Hilde45, I try to stay informed on the pro’s and con’s of using multiple subs. Would you be willing to share what these problems are, that you have heard of? I’m not trying to challenge or persuade you, and if you’d rather not, no problem.

Duke   
disclaimer: I’m commercially involved with a distributed multisub system.
Hi Duke,
Another very experienced member (building speakers over 40 years) on Agon mentioned that a second sub *could* cause interference or node conflicts. I was debating if I should buy another one and I was having trouble finding an identical REL model (528). His handle is @decooney To be clear, he wasn't saying I would but that I could and that if one was enough for my room and it wasn't giving me issues, I could relax on that front and focus attention elsewhere.
@hilde45, Duke,

Take this with a grain of salt. Varies by the listening room.  While I prefer stereo subs, it’s all room dependent IMO. Phase and frequency cancelation at varying lower frequencies "can" occur with two, particularly in uneven odd shaped rooms -or- in situations where you are not able to place a bunch of room treatments or not enough phase adjustment in the sub x-over itself. I try not to advise on this as its mostly due to an environment-room issue.    

I still run stereo subs now, and often times leave one subwoofer powered off based on my seating position [in my case, in a particular room]. If I run the same setup in a different room, with even side walls, no issue, dual subs works great. Again, more of a room problem and no guarantee until you try it either way, YMMV.  
I have a very good tube untegrsted in Ayon 
and a very solid Yamaha 2100 both have active preamplifier sections , SVS, or crutchfield sells a resistor adaptor that connects directly to speaker outs on the amplifier , it is more accurate 
and better Bass control 
if you sub has speaker inputs, like rel  has it’s a very good setup .
I use this which it has a + and - per channel ,you use awg 16 wire I use Neotech and crimp bananas to one end to go into the speakers ,the other side plugs into the adaptor board and there 
you plug in rca sub cables into the board ,the other side into the sub input , it’s sounds like a lot of work ,but it’s not and only
$30 I Velcro to the back of the audio stand.