Ah Njoe Tjoeb CD player

I bought this CD player like you hoping to hook it directly to my VTL MB 250s (input sensitivity at full volume is 1.6 volts and input impedence is 137Kohms). It doesn't work with these amplifiers at whatever voltage output I change it to. It may work with other less sensitive or lower output power amps but my experience is that even with the cd player at the lowest output level possible (where the sound quality is not that good) the volume is too loud for prolonged listening.
The unit itself is fun. The presence and soundstage are great and stand out from other CD and DVD players I have heard. Luckily Upscale audio furnishes the unit with two sets of tubes, one from the factory and one matched and burned in. The factory tubes really sound tinny, even my wife noticed.

We have had a number of people use the Njoe Tjoeb direct into amps, but there are times the volume may not go low enough.

Some amps take very little voltage to get them to full volume. The McCormack DNA-1 is one. You may need a passive attenuator for that. Available from Creek.

Or...depending on what your total budget is...get a super cool integrated. We have had great results with the Cary SLI-80 Signature. Recently we have also use the Antique Sound Labs. And for solid state Plinius, and Musical Fidelity.

Which one works best for you depends on your power needs

We have had a number of people use the Njoe Tjoeb direct into amps, but there are times the volume may not go low enough.

Some amps take very little voltage to get them to full volume. The McCormack DNA-1 is one. You may need a passive attenuator for that. Available from Creek.

Or...depending on what your total budget is...get a super cool integrated. We have had great results with the Cary SLI-80 Signature. Recently we have also use the Antique Sound Labs. And for solid state Plinius, and Musical Fidelity.

Which one works best for you depends on your power needs .

In regards to feedback on on tube swapping, I put a vote in for the Amperex Bugle Boy 6922/E88CC's. I tried the stock tubes and the Siemens 7308's that Upscale Audio recommends. I find that the Amperex has more midrange warmth than either. Of course, I am running a system with a Solid State Amp so additional warmth is a priority for me. If you have sufficient warmth (existing tube Amp) and are looking for a tighter bass, I think the Siemens would work well. In either case, I would recommend trying something in addition to the stock tubes.

One little thing that has really helped out is the inclusion of the CD remote. It fits my hand perfectly and it has allowed me to skip purchasing an expensive optional ($500) remote with my pre-amp.

The only thing I can think of that would improve on the current edition is the removal of the Marantz CD4000 face plate and add a custom AH! one. I know it is kind of shallow, but it looks kind of funny having all high end gear along with a marantz CD player. You are constantly saying "No, its a tube Mod - really!".