ARC D115 Mk2 vs ARC M300

I think Linnlp12 is giving wise advice. (Pay special attention to his comments about ruining equipment sound with cap and resistor change outs - it is a very real problem and you won't know it until it is too late; at the very least, keep all the old parts because you may want to put them back in.)

More to your original question, the D115 MkII is generally regarded as among the all time classics in the ARC amp line. The next step up for you might be a VT200 (or a pair of VT200s). The VT200 has a huge amount of authority and musicalness. Note, this is the VT200, not the MkII. In general, the 6922 designs have built a strong following among long time ARC users. If you need and can afford 2 VT200s you will be rewarded with highly impressive and enjoyable sound. Alternatively, you could start with 1 VT200 and then add the second one if you need it (starting with one would seem to be the preferred approach.)

For $ for watts and overall price for performance your biamped D115MkII / SP10MkII combination is going to be tough to improve upon. If you are happy with your speakers and source, I'd say it's time to attack room acoustics rather than put more $ into your electronics. My recommendation is to enjoy the music and forget the system improvement; the next step up from where you are is going to be an 80% or more increase in cost and it will bring a 20% or less improvement in the sound - and even that modest Return On Investment will only happen if you have an excellent room in terms of size, dimension ratios, room treatments, etc.

I have had much experience with ARC amps and also own and SP10, I know many have raved about the D115 but I never developed a liking for it. So think about this as you consider my recommendations. My favorite ARC amp was an unmodified D75A. It had an awesome sound until I "ruined" it with a cap and resistor upgrade in my "upgrade components for better sound" period in my life. I have since abandoned that philosophy. For the D75A, you also need a warehouse full of 6CG7 tubes if my memory serves me right. I think it used 16 of them. I have heard the VT100 series but while it sounded good, I felt there were better in line with my preferences(my opinions, no flames please) My experience with the SP11 is same as yours, owned it and sold it for the 10. With the right tubes, I have not found better. With so much gray hair on my head and lots of experience (I find music to be one of the main stress relievers on a day to day basis), I would suggest some amps you might consider. I cant comment on the BAT as the previous poster suggested because I have no experience with them. Here are two combinations for your consideration and are based on hooking them up with SP10 and combos of many speakers.
Cary SLM-200. Awesome, awesome tube amp and goes great with the SP10. There is a special synergy there.
Plinuis SA250 as solid state with one exception. If you use a very low output moving coil cartridge in an analog situation, the synergy is not there with the SP10. I find plenty of gain but not the magic. I dont know why this is so, but this is my experience. With medium or high output cartridge it sings. To me, this is the best solid state amp I have ever heard and owned (again, I am sure there is always better but not for me. These two amps caused me to stop dying under the ongoing upgrade disease). Cant say will never look or buy but right now I am really really satisfied. I LOVE both these amps and would not hesitate. They may be out of your price range but it took me 20 years to finally to be able to acquire these so there is hope. The wait was worth it.

Dmc. the sp-10 is a keeper.

Oh yes, you will experience much the same when you switch from the D115 to the M300 like you heard from the SP-10 to the SP-11. I too owned the SP-10 for 8 years and after hearing the SP-11, I had no desire to change.

The hybrid ARC products are not at all the same character as the all-tubed units the came before them. After the hybrid period, ARC returned to all tubed units with the LS5, VT130 and VT150.

I made the mistake when I went from a VT130 back to the Classic 150 (this followed the M300) hoping for more dynamic headroom when in fact all I got was a more analytical and fatiguing presentation and lost the great bloom of the VT130.

If you like the SP-10/D115 sound, but are tired of the higher noise floor of these older models, and/or having reliability problems, you can either look into the ARC models that followed the hybrids or abandon the ARC ship altogether for something like the BAT line.

John