TL/DR: The Rachael and the Sennheiser 6XX are a home run.

Finally got around to using the headphone output on my Decware SE34i.6 (Rachael) amp. It’s very good.

This Rachael is a natural match for the Sennheiser 650/6XX. Superb midrange from the 6XX / EL34 tube combo. Smooth, non fatiguing highs, very good sub bass and overall bass quality. Tight and controlled. The 6XX can sometimes struggle in the bass department when not amplified correctly. I’ll need to hook up my DarkVoice to do a comparison. But overall excellent synergy with the 6XX.

If your a bass head and like trunk rattling type bass the Rachael and the BeyerDynamic DT990 600 will be to your liking. I found the bass to be a bit boomy and loose for my liking. The midrange is a bit recessed which is normal for this headphone. The Rachael does a good job of controlling the treble which can, depending on amplification, be too much with this headphone.

The LCD-X is a non starter with this amp. IME planar and tubes don’t mix. At least not with the high impedance tube amps I have. Rachael, Willsenton R8, and DarkVoice.

The Rachael and the Meze 109 Pro are a good match. The bass isn’t as controlled as the 6XX, but no where near as bloated as with the DT990. That said, it’s a good match. Very listenable and non-fatiguing. The 109 are also always tops for comfort.