Is the Manley Steelhead still relevant?

There are many reasons why you may experience a "washed out" SQ with higher volume levels, in your system.  But keep in mind that a volume control is an attenuator; it is not adding gain as you turn up the volume. It is attenuating the output of the device less and less as you go toward the max volume.  The tubes and transistors upstream from the attenuator and which are responsible for "gain" are always operating according to a fixed set of voltages and currents, producing in the device a fixed signal voltage (until you get to the attenuator) with an inherent signal to noise ratio.  As you employ less and less attenuation, not only do you get more and more signal voltage to drive the amplifier, but also you are permitting more and more of the noise floor to drive the amplifier.  So, the end result is that you begin to hear the noise, albeit it is way down below the signal voltage by the same ratio in db that is inherent to the device.  As to why you feel the sound eventually acquires a "washed out" coloration, that is a complex question that is probably the net result of many factors.
A cathode follower is an active device that has very very low distortion and adds no gain.  Its raison d'etre is to convert voltage to current, which is another way of saying it sees a source (the upstream circuitry) that has a high output impedance and it puts out the same signal (no added distortion, ideally) with a low output impedance so as to drive the downstream amplifier.  That's one definition of a "buffer".  The buffer function is very necessary for any linestage, whether it includes circuits that add gain, or not (as in the case of the Steelhead).  The low output impedance is very advantageous for driving any amplifier (see also below). 


What fsonic and maybe Dover mentioned is another class of "linestage" that is completely passive in that such devices contain no active components at all (no tubes or transistors); they are basically just attenuators.  This can be done with a resistor network or (better) with autoformers or transformers.  The problem created is that you need to match the output impedance of the device driving the passive attenuator with the input impedance of the downstream device.  The rule of thumb is that the output impedance of the driver (in this case, the passive attenuator) should be about 1/10 the input impedance of the driven component, in ohms, and this ratio should be consistently maintained at all levels of attenuation.  (This criterion is easily met by the active CF to be found in the Steelhead.) It's not so easily or universally achieved using a passive attenuator. Therefore, passive attenuators need to be carefully selected and matched to the up and downstream components for good results.  Many aficionados do swear by passive attenuators as linestages.  I am not in that camp.

@drbond 

So, when you say the Manley linestage is just a "buffer", you mean that it's not amplifying the signal by adding gain, but just transmitting the signal unadulterated?

Yes.
The washed out sound at higher volume settings suggest the Manley is either unable to drive the Atmasphere power amp adequately, or the quality of the sound is deteriorating at higher volume settings due to the circuit topology in the Manley.

Adding an active line stage will fix that issue and may even benefit your other power amplifiers as well. I would encourage you to try some out if you can.

In terms of options Atmasphere preamps should work well and have balanced outputs as well as single ended to suit their amps.
Conrad Johnson linestages at the upper levels are excellent but only have single ended outputs available.
I would be wary of the Lamm preamps without trialling them becuase they tend to work best with their own gear - the LAMM phono/preamps have unusual gain structures and in real use tend to work best together.

@lewm

What fsonic and maybe Dover mentioned is another class of "linestage" that is completely passive

I did NOT discuss passaive linestages in my post, and do not like them, and they would be a waste of time in drbond's system.
How many times have I told you to read my posts more carefully.
I get sick and tired of you misinterprretting my posts. You are supposed to be a doctor - would you like me to post in Latin ??

My recommendation for drbond is clearly to add an ACTIVE line stage between the Manley and his power amps.

@dover 

Thanks for sharing your insight.  I didn't realize how much the input sensitivity of amplifiers influenced sonic behavior while using the Steelhead line stage as a buffer.

Regarding pre-amplifiers, would you suggest any other brands besides A-S or CJ, or are those as good as any? (As I noticed that you mentioned that you have an Air Tight in your system.). Is there something in particular that I need to look for in pre-amplifier specifications, such as gain, etc?  

Thanks!

@drbond 
Depends on your budget.
The reason I suggested the Atmasphere MP1 or MP3 is they are certain to have enough drive to match the the MA1's. I think you can get these with or without a phono included in the preamp.
The Conrad Johnson ET7 up to the Gatt have very good drive for insensitive amplifiers. A friend of mine has recently purchased the Conrad Johnson Gatt2 - it is stunning but of course it has a big price tag. It's a forever purchase.
Air Tight are also very good - the ATC2 has great drive and can be obtained at reasonable prices second hand due to being less well known.
Audio Research in general I find a little brash for my taste and I think the others offer better value.

How much do you want to spend ?