Cd player direct or through preamp?

Advantages can be better transparency,openess,detail.By eliminating a component from the signal chain you remove the extra signal path,volume control,gain stage.In the case of a passive control you eliminate the extra volume control resistor ladder or variac.Simpler signal chains can be benificial.The pre-amp gain& buffer stage can improve the signal for better component match en`route to amp(s).This can yeild better dynamics,control,pace.Passives and active allow more source components like a tuner to be run in system w/o cable shuffling.A preamp with phono stage explains itself,and a multi input passive can allow selection of tuner,phono with phono preamp to be selected by remote or manual switching.If you have a cdp with internal volume by all means try it out maybe post what your amp(s) and cdp are,asking if others found synergy in that combo first if you have doubts of buffering which I understand is of more possible concern in tubed gear.

As mentioned, if all you listen to are CD (in which case I pity you, but that's life) and you have the variable output you should be okay. Have you checked into the output impedence of your CDP, and checked to see what your amp prefers?

I think it's hard to generalize because there are a lot of factors to consider; output voltage of the CDP, input impedance of the amp, quality of the output stage and volume control in the CDP, etc...not even to mention personal preference!

My personal preference with the gear that I own/work-with tends to like using an active preamp. I've had experience comparing the Resolution Audio Opus 21 and Wadia 830 both direct and with-preamps, and in both cases found the with-preamp setup to be more enjoyable to me...more depth, dynamics, and musicality to my years.

Just my opinion...

My experience with my Sony 9000es Modwright cpd, with
built in volume control, it sounds best without using
preamp.

I had my Accuphase dp75v into a ML 32 pre into my Edge NL12 with my Kharma 3.2FEs. When I took out the preamp from the chain, I was shocked at how the sound improved. The flow of the music was much easier on the ears, with a more relaxed sound, but still full of deliniation of all the music's components. This was a big surprise for me because the ML32 had been a big step up from previous preamps (Threshold T3).