Engine
The 990 is a water cooled 75 degree V-twin. I've always loved the feel and the sound of a good V-twin. To my ears, it doesn't sound quite as good as a 90-degree V-twin like in a Ducati or Moto Guzzi, but good enough. I never felt the need to make it louder. The 1090R is based on the same engine, so it has the same V-angle.
The 790R is a parallel twin, but the crank pins are offset 75 degrees - KTM says it's a 435 degree firing order, but that's really the same as 75. I can't tell from the sound or the feel that it's not a V-twin. A parallel twin can be made quite a bit lighter than a V-twin and it takes up less space in the frame, so that's all good.
The 990 had high compression pistons, but I don't know what the compression ratio was. It was supposed to have 91 octane fuel. The 1090R was 13.0:1 compression. The 790R is "only" 12.7:1 compression. Gotta pay a bit extra for fuel for these bikes. Actual engine sizes for the three bikes was 999, 1050, and 799. KTM rounds off the engine size to the nearest *90.
Power
The 990 had great power. It was more than I needed, but just a
little more. The bike was not smooth at low speeds, like going into a 20 mph corner with the throttle off and wanting to feed in a little bit of power. I got in the habit of slipping the clutch to smooth it out in that situation. After a tune up it was better for maybe a thousand miles, then it had a hesitation there again. I liked the power band on that bike.
The 1090R just plain had more power than I needed. I rarely ran it to redline with full throttle. But the power was smooth everywhere and very good low down. Like every V-twin that I've owned, it wasn't really "happy" below 3000 rpms in the higher gears, but on dirt it was fine even under 2000 rpms. I would often chug along on tight stuff in second gear around 2000 rpms and it still had all the power I needed at that low engine speed.
I'm still breaking in the 790R, so I'm not supposed to exceed 6500 rpms. I could happily live with the powerband I have below that speed. It definitely has enough power for what I want. It also doesn't seem to like running below 3000 rpms, or maybe 3500. But off road it's totally happy under 2000 chugging along and if anything, feels more lively in the dirt than the 1090R did. I can't tell from the feel or from listening to the engine that it isn't a V-twin. In neutral, the engine seems to rev up quicker - or maybe "easier" - than any of my previous bikes. I'm sure I'll adjust to that over the next few hundred miles.
Suspension
After I got the 990, I adjusted the suspension to the settings recommended in the owner's manual and was happy with the behavior for all types of riding. I occasionally bottomed out the suspension when riding 60 mph on a dirt road with a dip, but "just barely".
I also checked the suspension settings on the 1090R to make sure they matched the recommended settings in the owner's manual. The suspension felt rather firm off road, especially compared to my XR650L (which is quite soft), but it worked well, so I didn't really try to mess with it.
The stock suspension settings on the 790R are definitely softer than the 1090R, but feel right to me. It's quite easy to get to the adjusters with the fork compression at the top on one side and rebound at the top on the the other side. I'm always of the philosophy that "if it ain't broke, don't fix it", so I don't know if I'll mess around with the suspension settings or not. One of the reasons I got the "R" model is for the better suspension, and it makes a difference in my riding. Plus it has about an inch more suspension travel and ground clearance.
Handling
The 990 works well off road. I've ridden with a guy who has a 2008 990 Adv just like the one I had, who rides it like a big dirt bike. I would have to be a lot stronger and have more riding skill to get away with that. I rode it on a lot of dirt roads and some single track trails and it handled them well. But the weight was always obvious.
When I bought the 1090R, I was expecting to adjust to it right away. On the street it felt right to me from the beginning and there was really no adjustment period. It was a lot like the 990, but slightly better in most ways. However, off road I had
confidence issues when I first got it. Part of the problem was that I hadn't ridden off road much in the previous year, so I wasn't feeling very confident in the dirt. I might have been the same on the 990 at that point. After a few hundred miles in the dirt the confidence returned and I rode many hundreds of miles all over the southwestern part of Idaho and enjoyed how it worked in the dirt. But I was always aware of the weight.
In the first 100 miles on the 790R, I was able to ride maybe 15 miles on familiar dirt roads. I also did a little bit of single track - just because. It might be that the dirt had been wet, then packed down so that there was no loose stuff on top as is normal around here all summer, but the dirt road handling and traction felt just great. Until I tried the little bit of single track riding, I was starting to wonder why I still need the XR650L. On the tighter narrow stuff it was clear that the extra 70-80 pounds compared to a dual sport still made a difference. I'll have to report back next spring as I get out more, so far the 790R feels way better in the dirt than the other two KTMs I had before it.
The riding position on the 790R, after adjusting the handlebar forward all the way, feels very natural to me. The riding position just feels right. The bike and I are one when I'm riding it.