The D-800 is sort of similar to the TT-800 with obvious differences.
My active 5-strings are only 9V units. However, they can push the amp into clipping the power stage and that did not sound all that great. I had to ask a few questions in some of the Mesa Boogie bass players clubs on facebook. I only got just brand references but nothing else. It was a good start though. Found the Darkglass Hyper luminal Bass compressor. Nice and transparent but will cut or compress the stronger signals to keep the preamp happy. When you use it with the passive modes (assuming you can turn it off) it will only limit the peak signals if they reach the limit. Now I can run my 5 stings and active 4-string, set the high gain, deep and bright boost switches to on without forcing the power amp into its limit mode. Here is reference for the compressor.
https://www.sweetwater.com/store/detail/HyperLuminal--darkglass-hyper-luminal-bass-compressor-pedal
As for the loudness of the 8 ohm speaker, it will not seem like much. I have run my TT-800 with all combinations. 1x15, 1x12, 2x15, 2x20. Just to hear the differences. Do not run the single 8 ohm cab in the 2 ohm switch position, you may damage the speaker. However you have an option if you add another cab for a total load of 4ohms. You can use it in the 4/8 ohm position for 400W or run it in the 2 ohm position for 800W. This is also mentioned in the manual. I was just looking at it for reference from Mesa's Website. Wanted to see how different the D-800 was before I make any comments.
What I found works best using a 5-string with the TT-800 is the following: two 1x15 cabs, one 1x12 and one 2x10. Since this need to be set at 2 ohms. you will be shaking everything in the home, including the dishes, glass wear, silver wear, and all things found in a typical kitchen. Items on walls in adjacent rooms may fall off. If you plan on going to ground shaking levels, prepare to protect any breakables that may vibrate off of shelves, furniture or countertops. I learned the hard way. The 4 string does not really need the 1x12 but does not hurt to have it.
Just the 1x15 and 2x10 will get you what you need for the 5 string but will seem to be missing something. When I added the 1x12 that was it. It has a breakup sound to it when pushed. If that is not your ideal, just the 15 and 210 would suffice. I tried two 15, not bad, too much low end. I needed that extra punch and midrange I get with the 2x10.
The D-800 is similar in some respects, the impedance setting also configures the output power.
Also, best to use a Speak-On speaker cable. I think the manual states the NL-2MP type for correct wiring. Refer to the manual for proper cable selection for the speakers. the 1/4 type TS speaker cable will work as well but leaves you with the potential for electric shock due to the connector housing is connected to the sleeve of the plug. Class D amps are different than typical guitar amps as they have an earth ground reference point.
