Thanks for the contribution, sfmatt.

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That's strange... it used to be in portables. I'll send a request to get it moved.

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The LM4562/LM6172 got me 17 hours, and was the one I stuck with for a while.

The LTC6241HV/LMH6655 sounded very similiar to the Predator but only got me 7-8 hours - weird. But, the highs were a bit smoother than the LM4562 or the Predator.

Will there be any different (in SQ) if I connect my D1 to the Compass?

I did not measure the battery life accurately but it seems on par with stock.

One big drawback with 2xLM4562 is periodic RFI with my cell phone. I didn't have it in my pocket at the time of my initial test so didn't notice but it's pretty annoying actually. I never had such issues with the stock opamps. Not worth reinstalling them but it will definitively be a consideration in my future rolling sessions.

greenarrow: I don't know for your DAC specifically but connecting the D2 to the headphone output of a RME9632 sound card yields a very noticeable improvement compared to the USB dac of the D2. If my laptop had a SPDIF output I would be very tempted by the D10 which I assume has better DAC performance through SPDIF than through USB.
It's too bad there don't seem to be any high-quality dacs that take USB inputs directly.

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The Pico DAC/amp is very good as a USB DAC/preamp, but the DAC-only version should be able to avoid any worries about sound coloration as it leaves out the amp section. Pico DAC-only: HeadAmp Pico Upsampling USB DAC - Pre-Order Now at intro price - Head-Fi: Covering Headphones, Earphones and Portable Audio

The D10 USB DAC is very close to the Pico via USB and matches it via optical (and with native 24/96 music can surpass it's detail). I fooled both Sherwood and Blutarsky into thinking they were listening to the Pico as USB DAC feeding my Woo GES and Stax O2, when it was actually the D10 via USB feeding it.

I'm back at it: I've been listening to 2xLM4562 for a while and have been very happy with the combination. Still I found the motivation to try to improve on it:
- I got my first desktop amp (a tube amp, the Jade) which I'm in love with. I ordered a few tubes and was impressed on how much they changed the sound signature of the amp. The Jade showed me how much better an amp could be and got me back into a rolling mindset.
- The D10 thread is many times more active than the D2 Viper ever was and there are constant improvements made. With the D2 being more powerful (150mW x 2 vs 100mW) it should be possible to make the Viper sound even better than the D10 (as an amp only).

So after a lot of searching and reading I ordered 2 new pairs of DIP opamps: LT1364 and OPA2111. It has been really fun trying a gazillion combinations but of course it greatly increased the likelihood of a misshap and I ended up frying one LT1364 Compared to the stock opamps or LM6172, the new opamps and LM4562 are considerably better. I tried some random combinations at first but pretty quickly it became clear that there were only 3 contenders: LM4562 and the 2 new opamps.
Both OPA2111 and LT1364 are airier than LM4562 and they have a better soundtage too, both more or less at the same level. Both also offer a deeper (lower) bass.
OPA2111 has the most bass of the lot, a tad lower than LT1364 (but really not by much) but it also has a mid-bass hump. It is also the brightest of all. 2 of them together and the sound becomes really too bright. It doesn't feel balanced even if better at higher volume but at low to medium volume the high end push is off-putting (to me at least).
LT1364 is the most natural of the 3 with a good bass extension, good treble but not bright and an excellent midrange. 2xLT1364 sounds really good. It never feels artificial.
LM4562 is the most different of the lot, it's not as open, doesn't go as low and the midrange doesn't sound as rich when compared to the other 2. Also sometimes it can sound "digital" or "plastic". Still it has some characteristics I prefer: the bass has some good punch (the other 2 barely), the treble is just right (in between the 2 others but somehow the lest aggressive) and it is more detailed (textures of violins or bass are richer for example).

After trying all the combinations the best ones use LM4562 because it combines well with the other 2: it brings them the impact they lack for the bass and gives them that extra bit of detail while keeping their extended range.
Mixing OPA2111 with the others tames it a little bit but not enough to my taste. I find the sound aggressive, it's ok for a while specially with LM4562 (the bass is nice) but quickly tiresome.

In the end my choice went to LM4562 (ground) + LT1364 (main): excellent soundstage, good extension, good detail and good impact, sounds excellent at low volume (important to me, I spend many hours a day listening to music through the Viper). The only drawback is that it is a little bit on the bright side, more so than either 2xLM4562 or 2xLT1364.
Note: curiously the reverse combination (switching them around) doesn't work very well, the bass is overemphasized and feels slightly distorted.

What next? I am not aware of any DIP opamps that woud be either a step up or sideways towards a signature I would enjoy more. As for non-DIP ones there's a few I'm curious about (ex: OPA2211A) but without hope for much improvement over what I've tried so far (if any) so I won't bother. Still there's a last one I really want to try: 2xAD797. I need to find a fellow head-fier to solder them on a brown dog for me though.