Is the Manley Steelhead still relevant?

For your CD player or any other device that puts out what is generally considered a high output voltage, like a tape recorder, a television set audio output, etc., the gain added by a formal line stage is completely superfluous. The output voltage of your CD player is fully adequate to drive an amplifier.

@lewm 
Thanks for the detail.  I may try running my CD player without a pre-amplifier just to find out how it sounds.  (Unfortunately there's no volume control on my CD player, so I'll see how loud or soft it is.)  

So what you're essentially saying is that a pre-amplifier may be superfluous for CD players, and are essentially only volume control devices, which may also add some "color" to the signal?

the gain added by a formal line stage is completely superfluous.

What Lewm is referring to is the standard set by the industry back in the early 80's of 2V RMS for most source components. At the time, turntables were all but extinct. 
I disagree that a preamplifier is "superfluous". The better word would be, at best, "optional". 2V RMS is plenty for virtually every amplifier in terms of making full use of the amplifier's power. But for starters, what does that have to do with such features as input selection, balance, mute, phase inversion, etc? 
But more importantly, a great preamplifier does other things-it creates a sense of drive, propulsion, dynamics, and even space that is absent using a passive volume control at unity gain. Read any of John Atkinson's reviews of top tier preamplifiers over the last five years and he makes note of how he has come around to this realization after once thinking that a passive volume control offered a "purer signal". 

I was referring to GAIN only, when I wrote that a linestage is superfluous. Obviously it’s needed for attenuation (if the phono has no volume control) and for input selection, at a minimum. Sheesh!

I would not connect the full output of
a CDP direct to an amplifier. In certain cases you might damage your speakers.

@drbond
Nobody has asked you the key questions -
What power amp are you using ?
For your current listening how high do you have to go on the Manley volume pot ?

From your other posts - you appear to have Atmasphere MA1 amplifiers.
These are an easy load to drive ( 100k ) but I note that they are less sensitive than many amplifiers ( 2.8V to full power ).

Your Manley linestage is just a buffer - no real gain.

So to answer your question on what a preamp would do, -
If you find that you have to turn the Manley volume pot past halfway for normal listening then adding a linestage with gain means that the device driving the power amps will be working less hard - this may result in more headroom, and a more relaxed sound overall.

I have found that despite DAC’s/CD Players usually having enough output to drive power amps direct, the insertion of a high qusality lineage often brings about a more natural, unforced presentation - even more transparent in some cases, despite the additional preamp and cable.

For example I regularly listen to an Air Tight system with Tannoy Autographs. Even though the power amps are very sensiitive and the DAC a stereophile class A product, we still prefer inserting the matching Air Tight preamp between the DAC and power amps, even though the additional gain is not needed.

If I were in your position and you find you are running the Manley at high volume for normal listening, then rather than add a line stage, why not investigate an Atmosphere preamp that includes a phono stage. Less boxes and less cabling.  The full function Atmosphere preamps are well regarded with excellent phono stages.

I would not be surprised to find that you find some significant improvements by having more drive to the inut of the MA1 power amps. Perhaps see if you can trial an Atmasphere MP3 or MP1 full function preamp ( with phono ) you will quickly get the answer you are looking for.