Here's something odd from my Shanling M2s listening... I actually have the M2S set to Line out, as it uses a different opAmp. And frankly it sounds quite good! I'm really really impressed! I mean there's no Head Room, as even with the Line Out setting I can only get up to a calibrated 66.1 dB Pink Noise Measure, which net's me peaks right around 87.8 dBs with the Eagles Live Rendition of Hotel California from the Hell Freeze's over Album.
Still, despite the utter lack of head room, the Line Out opAmp drives the HE 4XX REALLY well! Noticeably better transient response than with the Variable Head Amp topology their using, speaking of the variable amp, Though the tricky part is Volume Matching... I get an average of 66.1 dBs of pink noise calibration with both, but the LO setting will peak a little bit higher... by about .5 dB's so with actual music it's on average about .3 dB's louder. I typically like to do impressions within .1 dB volume matched on both pink Noise and Peak's with real music, but I don't quite have the option with the m2S's line out
Either way, my pico power and Ember II also peak around the same .2/.3 dB higher than the M2S Variable Head Amp Loop, so I suppose in the end it's all moot. Technically if I amp the HE 4XX up to peaks of 90 dB's it sounds the best! But I've kept my impressions at an average of 87.8, with some of my amps peaking up to 88 dBs,
Switching over to the HM 601, I can say the HM601 like the Ember II Pico Power and M2S Line Out, peaks at 88 dB's while pushing out 66.1 with pink noise, the line out of the HM 601 is colder than what the Head Amp maxs out to... which is nice actually,
In terms of sound... WOW, I recently choose to switch over to the M2S as my primary entry level dap... and that said, the Hm 601 still impresses me. It's got better resolve than the M2s, with a smoother top end and a much HEAVIER low end. But that heaviness in the lows soften the vibrato of the big drum in the intro of this track. So while the 601 resolves better in the mids/highs it's a bit softer/diffuse in the lows... still the added weight is a fair enough trade off. The mids are also much the same, a trade off, the 601 with the HE 4XX resolves more transient information, more detail and vibrato on the trailing edge of both the vocals and the guitars with the hm601, but it lacks some of the tactility that I get from the M2S! Given how warm the HE 4XX already is, I doo prefer the more tactile presentation of it with the M2S, the added details and mid range sweetness on the 601 are nice, but not something I'll notice as much when I'm listening on the go. For an entry level portable, something I carry with me when I travel on planes or go any where that's super noisy and "hectic" I'd rather have the tactility over the marginal improvements to resolve, in a noisy environment I'm going to notice and enjoy the tactility more from the M2S than the quite details the 601 brings
Either way, both pair nicely with the HE 4XX, which I like!
I gave the JDS Labs EL Dac a listen with the Vali 1 and HE 4XX, I fed the EL Dac with Coax from my Shiit Etir, when Pair'd with the Vali 1 the combo brought a very heavy well resolved low end to the Planar! It was really a lot of fun! I usually stick with Hotel California when I'm evaluating how an amp/dac pair with a headphone, but I went off the rails an enjoyed some EDM with the Vali 1 Combo! It was really a lot of fun! Resolve and imaging were wider than deep with the Vali 1 EL Dac combo, and the mid range had a nice polite tactility in both the low notes and further up the neck, the vocals were beautiful too! In Terms of resolve, the EL Dac Vali 1 combo were... comparable to my HM 601, really the EL Dac and Vali 1 had all the strengths of each of the portables I compared it to! I still want to pair the EL Dac with my Pico Power and see how the Dac/HE 4XX scale with a better amp