Midrange hiss....VERY slight...with my Dared pre

No tubes do not do this. You must have bad tubes in it or other problems with the unit. I suggest you have the unit serviced. Or try different tubes in it.

Some tube preamps have a high noise floor. Some tubes are not 'low noise'. Some amps have high input sensitivity. Some speakers are very efficient. Any or all of these things, especially when combined (two or more), can cause a sound called "tube rush". Proper matching of components is essential to avoid this noise. If you list the names and spec's of your other stuff someone here can probably diagnose your problem.

What type of tubes are in this unit now? Do you know how old the unit is or really how many hours the tubes have on them. What is the efficiency rating of your speakers and what brand? Single driver or multiple?

Do you hear it when nothing is playing? Can you turn the volume up high (with nothing playing) and hear the hiss better? The way you describe it, it sounds like certain frequencies 'cause' the hiss to appear, or like the hiss is 'riding' on certain frequencies. Good old 'tube' hiss is usually constant and can be heard in the absence of a signal. Replacing the tubes (if they aren't too expensive and there aren't too many of them) is the quickest way to narrow the problem. But if it's coming from both channels (you dont say),then, unless there is a tube rectifier, it's some component common to both channels (and so probably not two bad tubes). Good luck. (Ask the seller about it)

I have the SL-2000A.Its the 2005 unit with the upgraded auri caps,silver wiring,nicer cosmetics,2 outputs.
My dared is dead quite,low noise floor.Never a hiss or any odd sounds.
I have vg NOS tubes in it.
This preamp is very sensitive to tubes.
I would start with your tubes as the problem.