Okay, here are the results of my double vs triple EL2001 buffer tests. It was conducted with two identical portable META42 amps, built within a day of each other using parts from the same batch, each with slightly less than 1 hour burn-in, and both having the AD8620 op amp and using fresh Plainview batteries. The source was a Panasonic SL-CT570 pcdp using the line out to a Markertek cable to the amp.
The expected and actual benefit was in the impact of the bass. To evaluate this properly you must match the compared levels carefully, as typically the louder unit will sound like it has the bigger bass impact. I adjusted each amp to have as close to the same gain as possible. This is harder to do than say, as the volume control only has a finite resolution and there is also some channel inbalance in each unit. I adjusted the outputs of each amp to be on the average of within 0.5 dB of each other, and later changed the slight error in favor of the other amp and repeated the listening tests. Here are my findings with both low and high impedance headphones:
Low Impedance phones, Grado SR-80 and Audio Technica W2002:
There was a noticable increase in bass impact, depth and tightness with the triple buffers. When the volume slightly favored the unit with the triple buffers I thought the impact was quite impressive, but when the double buffer unit had the slight (within 0.5 dB) volume advantage the triple buffer's advantage was still there but more modest.
High impedance headphones, Sennheiser HD580 with blue Clou cable:
I heard the same thing as with the low impedance headphones but to a lesser degree.
In conclusion: going from double to triple EL2001 buffers does increase the bass impact, although low impedance headphones will show more of a difference than high impedance headphones according to both theory and my somewhat limited experiment. There is a price and battery longivitity penality to pay for the extra buffer per channel but is is fairly modest. For me it's probably worth it (although not a must) for low impedance headphones, and for high impedance headphones it would depend on the value of longer lasting batteries.
Some people have gone much higher than just triple buffers, but three is the maximum that will fit into the Serpac H65 case.