2.1 / 2.2 Request for Amp Suggestions

Recently, I purchased a Rogue Audio Cronus Magnum II and a pair of Magnepan 1.7s.

There's a noticable gap in the bass that I'd like to rectify. I don't use the system for TV/movies, though I probably will at some point. I prefer accuracy over thunderous response and am looking at the Magnepan DWM panels. My concern is that the power in the amp is definitely not enough to drive the DWMs, not to mention that the amp is already wired for the 1.7s at 4 ohms, so mucking with the impedance will probably result in blowing fuses and tubes.  The room size is on the large side. The 1.7s and amp seem to be matched right now, both for power and resolution, especially after rolling in a Telefunken preamp tube and getting the 1.7 placement pushed out to nearly 5 feet. Again, I'm absent the bass I want and am unwilling to muddy the 1.7 sound by pushing them back toward the walls. 

With this in mind, I'm thinking about using the preamp output on the integrated amp and connecting it to a secondary unit. What kind of amp should I use? I don't need an integrated amp as unit #2 and am struggling with the price of a Rogue Audio M180, even used. What would you do in this situation and why? 
I would get real subwoofers, “complimentary woofers” is just silly, just asking for phase cancellations and comb filtering, plus they are inefficient and low-impedance, so just adding strain on any reasonably priced amp. HSU VTF-2 MK5 or a Rythmik L12 for instance would be much better, or SVS if you want to buy from BestBuy.
Thank you!

A 12" driver seems large for the gap I'm trying to address. I'm saying this because I'm concerned about sloppiness and suspect I don't need reproduction down to 20hz. 40hz is ample. Does this change your suggestion?

Would this be addressed by using a set of 10s? Thanks again!

I would go with one of the JL powered subwoofers.  According to JV of TAS, the JL subs can match the speed of the Magnepan panels.  That will fill the hole in the bass
If you use the preamp output of the Rogue to use another amp the amp section of the Rogue will have no load. Tube amps need a load connected. If you use a subwoofer with an integrated amp you will have to run the speakers full range without the xover option. SVS has a 45 day return policy so you can try a sub and see if that does the trick!
Driver size has no correlation to being slopppy or tight, a Rythmik F18 (18”) is super tight, low distortion, etc. any subwoofers from HSU, PSA, SVS, Rythmik, and JTR are good. JL Audio makes good subs, but overpriced, a $2500 JL Audio sub performs (and measures) similarly to an $1800 Rythmik sub. All brands I mentioned are also started/owner by Americans and their customer service are great, and all or at least most have their PhD’s in mechanical or electrical engineering, they know what they are doing, Rythmik for instance has many pages on FAQ’s of their performance, entries on phase matching for perfect integration, etc.
Thanks. I'll still be using the line outs for the Magnepan 1.7s. The preamp out is meant to be used in conjunction with the line out and not in place of it for the reason you mentioned. 

I'll check out the SVS, just so I can get a feel for the sound quality. 

Has anyone ever used the A120s that Speakercity USA used to resell? They look very similarly spec'd to the Rythmiks that @mzkmxcv (sorry, tag isn't working) mentioned. 
Preamp is o.k. Use a poweramp from about 200 W or more. Use the good cables: to much silver is detailed and precise in high and mid,less deep in bass. Use copper wire or Rhodium (even golden plugs . Do’nt use a extra subwoofer: you gonna change  your music. I prefer for a power-amp.:Gamut, Ear,  McInt.,Pass ( but he is not so good in “mid”). And powercord is very important.
Also a good combination.:preamp:tube;poweramp.:transistor. You have to find a good match. Good luck looking for the right soltution.
If your looking for the ability to optimally locate your main speakers and still have an independent level of adjustability to integrate with your current speakers to your taste and more importantly your room you might want to compare the setup procedures for powered DSP equipped subwoofers.

Non comparative subwoofer reviews and sub reviews in general may have little bearing on your environment. Like most things audio unless you've actually experienced it beware of the claim of perfect integration.   

Consider a subwoofer purchase as an audio investment that will provide you service now and into the future to complement even the most exotic so called full range main speaker purchases.  

Currently the flexibility and quality of DSP adjustability varies. I'd suggest by studying the setup procedures of two top manufactures offering their second versions of subwoofer DSP, JL Audio's Fathom V2 series and their CR-1 crossover along with Velodyne Digital Drive Plus series User's Interface and Manual/s. Have fun with it.  
Thank you for this advice. I suspect you are right. I recall my father saying the same thing about the Velodynes self-correction capability years ago. That's the same DSP circuit you're referring to, right?

I wonder if the DSP has evolved enough to detect interference frequency off at the cross over points.

dunham_john OP4 postsI wonder if the DSP has evolved enough to detect interference frequency off at the cross over points.
I'm not quite sure whats meant by "interference frequency." I believe Velodyne began auto processing with their Digital Drive Series and now market Digital Drive Plus. Crossover points can be automatically or manually set in both. 

Velodyne Room Optimization Auto equalization using their supplied test tone disc, calibration microphone, and their software installed on a laptop will visually illustrate the room pre equalized response at the mic location from 200Hz -10Hz of the main speakers.

Running the Auto-EQ will automatically adjust the Setup preset levels from 100 Hz-10Hz as you watch, it also adjusts the subwoofer level, crossover, slope, and phase during the Auto-EQ optimization process. The program will end then reboot installing the setting into one of six presets that can be accessed by the systems remote control.

Manual EQ Optimization can be accessed to further adjust any of the nine parameters and saved into remaining presets. Reading the instructions initially seems daunting but once you've made an adjustment its actually quite simple to fill all the presets with slightly differing parameters to your taste or use the factory settings and all done at the listening position.

Why would you want to change a subwoofers EQ?
When going from Puccini to Prince or if you simply need to change the polarity for a miss recorded kick drum.
Some recordings need very little, if any, sub. Remote control down or off.