The J pipe clamps on either side can be loosened and the J pipes moved to lengthen or shorten them. This is what changes the resonant/drone frequency. The two Solo forum members, Steve and Vince, if I recall correctly, advised me that the factory setting is 9 1/4" in length, from the elbow to the clamp. With my Solo catback install, I get a whopping drone @ 1000-1250 rpm. Unfortunately, 80% of my driving is on rural and semi-rural roads, going 30-50 mph, ultimately in 5th and 6th gears, so it seems that, most of the time, I'm right in that rpm band. Oh well, it is what it is. There is no drone at higher-than-1250 rpms.
Have you adjusted those J-Pipes at all? Just because the 9 1/4" is where they come to you from the factory, doesn't mean that position is best for your vehicle. Adjust the pipes in 1/2" increments and fine tune them. I have zero drone in my car. I started with the Axle backs, Went to the catback, now I have the catback and Kooks LT headers. No drone in all three cases.