Mesa Boogie Subway 800+ vs AG700. I play blues rock with improve....so think about early Led Zep, Allman Brothers Band (early) and the Grandfunk Railroad "Red" album. I also am a 2x15 bassist. Which one of these amps would suit best?
I don't have the D800 but have played with them here and there. Instead I have a TT800, as well as a Tone Hammer 500 and a GK Legacy 800 on hand. The Tone Hammer controls are unlike any other. Lots of threads about that here on TB.
What I'm hearing is that you want thick, "tube-y" (insert can of worms) goodness and warm gritty drive. I can't think of a better description for the Tone Hammer heads than that. It's like playing a warm hug that wants to crush your ribs. It has been said by many others and myself that the Tone Hammer sounds more like what the mental archetype of a classic tube amp should sound like than most actual tube amps. It's crazy how they've built so much character into a non-tube pre class D amp.
The caveat is that the Aggie TH will NEVER get squeaky clean or bright. You CANNOT put back in with EQ the top-end airiness that is naturally voiced-out of that amp. (That's why I have the Legacy, haha!) The D800 will do more things better than the Tone Hammer will. But the Tone Hammer will give you that classic Grand Funk funk by the dumptruck load unlike any other Class D amp, imho.
Read up on the Tone Hammer Gain and Drive controls before you go try one out. I truly feel that a large percentage of the negative reviews around the TH are built on two things: 1) The user just not quite understanding those gain and drive controls. Their function is counterintuitive to their names, and it threw me for a loop as well. Once I read up on them and actually understood what was happening from an EQ/drive perspective, it became super easy to leave the EQ flat and just use those two knobs to quickly get to to where I wanted to be for a certain sound, then a smidge of EQ work to get you the rest of the way.
2) And most importantly, the user was probably looking for a bit more of a modern or brighter sound. Those are things a Tone Hammer just won't do. Ever.
A note about cabinet matching with these amps. I do believe the D800 will sound good with more cabinets in general, as it has less of a native/baked in sound. You can easily EQ in or out some top end to account for either aggressive or lackluster tweeters or upper-mid range response of a cabinet. You are starting closer to neutral in terms of overall response with the D800. This isn't about overdrive or distortion, but the native in-out sound reproduction of the preamp and poweramp, imho.
The ToneHammer, on the other hand, can turn to mud if also paired with a really dark/warm cabinet. If the cab design already stinks at upper mids, especially, you are TOAST. Note definition disappears and you are forced to just crank the volume to hear what you are playing. That's no fun for ANYONE, and leads to the headaches (literal) that I think someone else mentioned further up. I absolutely love pairing the Tone Hammer with my Mesa Subway cabs. Doesn't matter if it's the 115, 210, or both. It's just a delight. On the other end of the spectrum, I had an Ampeg Heritage SVT410HLF cab for a number of years. It sounded great with any neutral or bright amps, but that cab and the Tone Hammer just became muddy earthquake city.
I feel like the AG700 and D800 amps are closer to each other than the TH700 and D800. The TH is out in its own field when it comes to class D amps. Up to you whether that's what you're looking for or not.
My experiences with with the Tone Hammer, Mesa, and Legacy heads, which cover a lot of different ground, drove me to change my approach to cabinets. I now only have fairly neutral, powerful, cabinets such as the Subways and some GK Neo cabs. It's all a matter of personal preference and approach, but I have come to the place where I want to get the specific tone I want from the amp I choose, and I want cabs that will work well with either end of the spectrum.
Good luck on your search!