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

Let me start by saying that I've never owned a Tube preamp. I bought this piece 2nd hand, it has 'upgraded' tubes (installed by the previous owner, who seemed to know his stuff). The model is the SL-2000A. Anyway, after listening I noticed that there is a rather prominent hiss in the midrange. It sometimes is VERY unnoticable...but when the sound is playing in a certain range (perhaps around 2-5khz) the infromation is played with a slight hiss. Is this just what tubes do? Any help would be GREATLY appreciated.

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.
Wow, thanks for the responses fellas! Here's the rest of my setup.

Sony SCD-CD595
Signal Audio Cables
Dared Pre
older pair of kimber cables
Marantz SA6100 (solid state amp)
Epos ELS-303's

I'm thinking that it is the older pair of kimber's that perhaps are shot. The amp has almost zero noise if I turn it up half way...which is a solid 1/5 of a rotation high than I EVER turn it up while listening. At the present moment I'm one shelf short of a dedicated shelf for the Dared Pre...and it is resting atop my cd player...could this possibly cause such a problem???
If you can't hear tube rush at normal/high volume listening levels, its NOT a problem! Even more so if you only hear it when you've got your ear within inches of the tweeter/midrange. Many fine units both SS and Tube, operating properly, will reveal their noise floor depending on the input sensitivity of the amp and the efficiency of the speakers.
Newbee...thanks for the input. So, my very modern SS Marantz amp may be too sensitive inputwise for the dared pre??? I think that the epos are around 88/89 db/w.
Interestingly it may be just that and new has nothing to do with it. I have two new amps, both with high sensitivity (.5v and .8v)and two older amps by about 12 years which have much lower sensitivity (1.5V), some of the older ones even go down to 2.5v or less. Higher amp sensitivity makes the pre-amps job easier and facilitates the use of passive pre-amps for those interested. The speakers at 89db are moderately efficient.

Personal story - Had some Acustat panels with an 82db. With my SP10 and the matching ARC 115II amp my system was dead quiet. The I got some Quads - only 4db difference - all of a suden I could hear noise. Then I got a new amp with higher input sensitivity (.8v) wow - did I have noise now! Thats when I started to paying attention to low noise tubes in my pre-amp. Thats how it goes. Ask someone who uses high efficiency horns about noise! :-)