IDK your elevation, but I'm running without an airbox, UNI filter, modified end cap on a stock muffler, and the OMR carb mods. At about 1,300 feet elevation, my DS runs great with a 175 main jet. 185 jet with a stock airbox and questionable muffler just doesn't sound right in your case.
Tractor muffler could be the culprit. Putting a stock muffler back on with a different endcap can be an affordable alternative to an aftermarket muffler. I did some searching in this forum as well as others that cater to the DS650 last year, and found some threads that support buying an aftermarket end cap that replaces the two turndowns of the stock end cap with a single open pipe can produce more hp and sound better/louder. Some even had dyno results to back it up. I could be wrong but I think it was even THE STIR that had an excellent thread where he documented dyno results of the stock endcap, and multiple fabricated end caps and compared the results.
I wanted to go cheap. I wanted to keep the stock end cap with it's spark arrestor, but cut off and replace the two down tubes with something that would flow better. I went looking on fleabay for a end cap for aftermarket mufflers that would fit on the stock end cap after cutting the tubes off. I got lucky and only spent about $20 for a titanium FMF end cap, and stainless rivets to mod mine. The butt dyno tells me it's running better, and the sound is more aggressive and deserving of the 650 engine. Here is my thread if you want to check it out. http://forums.atvconnection.com/can-...d-cap-mod.html