Very simple answer: After 3-4hrs., the content of your small intestine has moved through the cecum into your large intestine and progressed down low because of the multiple enemas you had. This is the reason gastroenterologists have colonoscopy patients fast for a long time as well as using PEG solutions. I tried enemas 'til clear before a scoping, but ate until 9:00PM the night before, and my Dr. said I was not clean even though I'd done the multiple enemas early that morning.
Jimmy has the correct answer here.
The answer does NOT lie in your enema position or what tube or nozzle you use, especially after running clear after 5-6 enemas. What you are eating will continuously come down the pike, and this will be more noticeable with more roughage in the diet. A person on a low residue diet--high protein/low carb/low vegetable diet/or who fasts before playing will have much less of the problem. Furthermore, a dildo pumping in and out will also stimulate peristalsis. The series of enemas you take are good. Maybe use 3 quarts instead of two, but that should clean you out well.
The important thing is to stop the food with all the roughage 6-10 hours before your enema series if you plan a heavy anal sex night, since material coming down the intestinal tract seems to be a problem for you--you appear to have what is called a "rapid transit time", so you have to stop eating hours before your enema series.
Furthermore, DO NOT USE SALT, glycerine, epsom salts or anything else because in this case, you WANT any left over water to be absorbed, not to have anything in solution that might draw any water into the colon or have any residual laxative effect. And you don't have to worry about not using salt either, you'll be fine.
I use 8-18 inch long dildos on my husband and he uses them on me, and we don't any problem with brown matter on our toys.