The Best Preamp is no Preamp?

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.

That’s the only real fun to be had in audio.  Trying something new is a very good thing as long as you don’t blow up your gear. Keeps things interesting anyway. I’ll try it sometime with my Node2i. 

Everybody talks about high quality preamps, which makes over-all sound good. Yes, the pre MAKES (modify) poor sound from poor sources to be good. That means the pre does change the original music signal into slightly different signal. The modified signal would be good to our ears. Do not praise the preamp that makes poor source better. Just blame the poor source. If you have good source (well-recorded CD for example), you can enjoy the music much better with no preamp. Why does tube-based gears make good sound? Just because the tubes tend to add some distortion to the original signal, and the added signal makes our ears pleasing. The added signal is usually even-order frequencies.

Shorter speaker cables make the over-all sound better? I have to laugh at the guy who says that. Inside the speakers, there are very long coils. Due to limited space condition, the coil's physical shape can not be better than that of speaker cables. Compromised long speaker voice coil and very short speaker cable? Usually, the speaker cable length does not matter very much unless the cables run longer than 50 feet.