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.
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.