The Best Preamp is no Preamp?

I have an MSB Discrete/MSB Premier Power Base that I run direct to my Pass Labs 250.8 (with Wilson Audio Sabrina X speakers).  I am a fully digital guy, FYI, so I don’t need analog inputs, although my wife recently commented that she would like to hear some of my vinyl on a turntable for nostalgia purposes.  I too have been wondering about whether to get a tube pre like an Audio Research Ref 5 and whether it will add anything better to the sound. It seems counterintuitive that putting a bunch of wires and circuits into the signal path actually will improve the sound, but seemingly, lots of people with good systems claim that it is the case.  I have an open mind and will see if I can try something before I buy one, but is there any explanation for how putting a preamp in the signal path when you have a fairly higher-end DAC can make an improvement?

 I have an old Phase Linear 400 power amp. I can and have ran that directly from my CD player to my amp bypassing the pre-amp. Of course having volume controls on your power amp is a must.

A pre-amp gives tone controls and a master on/off switch, remote usability but does not amplify or change the signal, a passive component really.

I  have tried the dac direct route in my office system.  In the office I am feeding Emotiva XPA 1 monoblocks which drive an older set of VanL quattros.   I tried my Cary 200T Dac direct and the sound was Ok.  I moved the 200T back to another system with a Cary preamp,  well broken in Odessey Kismets monos, and highly Modified VanLs,  the sound is phennominal (to my ears).  

 I have subsequently gone direct from an RME ADI DAC 2 FS into the Emotiva XPA 1s in my office and because of the myriad features of the RME it works extremely well direct to the monos and allows me to compensate for room irregularities in my office / Qi Gong studio.  That said I will look down the line to introduce a little tube coloration into the office setup.   As stated by others it is a matter of taste and system synergy. Each element: cables, wires, power all, add or detract from the overall effect. 

but is there any explanation for how putting a preamp in the signal path when you have a fairly higher-end DAC can make an improvement?

Yes. The explanation is offered earlier on this thread.