Adding a sub woofer?

@ieales, absolutely. I think top model JL Audio subs may be the best available and their price reflects that. To my knowledge nobody makes a stand alone digital bass management system. The most sophisticated versions come with full room control units/preamplifiers. I must say however that the current units are easier to set up than the TacT units were. Almost any audiophile should be able to get decent results. The hardest part is setting up the microphone in the right place. The down side is the additional expense.
Having toiled with subwoofers since 1978 I can say unequivocally that you can not get the full benefit of subwoofers without a complete 2 way crossover,  be it analog or preferable digital. For those unfamiliar with this, the crossover goes between the preamplifier and the amplifiers. All connections are line level. Using speaker level connections drops the impedance to your amplifier decreasing it's damping. It is not acceptable.
Your money would be best spent upgrading your system before getting subs. 

As an REW alternative, the ML subs have an extra cost accessory called Perfect Bass Kit which includes Anthem Room Correction.

Pretty good results can be obtained with the PBK provided one does a bit of "book learnin'" with a room simulation application and a little common sense.

Hi Ryan,

Depending on how large your room is, what types of music you like to listen to and how loud you like to listen to it, a single PSB 250 or even two maybe too small.

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

 
In this case the pre level Inputs would allow you to use the PSB 250's low level input and output connection scheme that limits the level of the low frequencies reaching the mains. This setup would give you the most control and should be far easier to integrate than the other hookup methods.

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?

I did not read the owners manual all the way through but at the price point of the PSB 250 I highly doubt that there is any processing happening so it should be a clean pass through to your mains with the exception of the limiting of the level of the low frequencies below your chosen crossover frequency.

You may also need to plug the ports on the RP8000F's to get the best results. 

I noticed that the RP8000F's can be  Bi-amped. You may get satisfactory results with this option also if you can control the input gains of your amplifiers.  

Thanks Rusty - helpful. And thanks all for your comments - plenty to dig into here, lots of homework. I appreciate all the thoughtful ideas and suggestions.

SVS SB3000! I have this and a Sunfire HRS12 that seem to play very nicely together - both musical, accurate and DEEP...