What's your setup with the pre90, what dac and amp did you use? I'm considering getting the pre90 too but it'll be for a few months or next year.
I have a Topping D90, with a buckeye amp to Q950s. I have a Topping A90, so I tried that in pre-amp mode into the Q950s and I noticed a marked improvement so I decided to get a Pre90
Topping D90 alone has a weird behavior, the sound sounds great already but there are certain items that lack omph, and lots of lacking in terms of string plucking, spacial feeling. I didn't know the sound was flat until I plugged in my A90 (which made it slightly better). The Pre90 made is WAY better.
After adding the Pre90, HOLY COW. I turned out my favorite scene in Inception, the one where the dream collapses and the plates explode and such.
HOLY COW was the feeling, like I could hear a plate smash and exactly know which direction it came from. Before, the dynamics didn't feel so high.
Sometime the Pre definitely helped with.
Other sounds with strong guitar, and strings, and vocals really helped, also the audience now sounds far away, not so muddled with the music.
Something about it helps the overall sound.
Also the volume is very mechanical, it makes an audible tick tick tick when you adjust it, which I absolutely love.
Basically what the Pre90 did was bring out the spacing on all the notes, so it isn't so muddled (not sure how to explain it scientifically). It beefed up the highs, and mids, but kept the lows as is, clarity definitely increased throughout. This could also because I set the Topping D90 to pure DAC mode.