Midrange and mid-bass a bit thin? I don't think so, nor has that ever been mentioned in any of the multitide of other reviews and posts on this dac.
As far as changing op-amps or other mods, I have yet to see any posts on this. The chief engineer at Benchmark specifically cautioned against changing op-amps.
The original question was a comparison against the top name dacs:
In S'phile (5/04), John Atkinson compared it against the $17,500 ML 30.6, which weighs in at 80 lbs. He said that at first he found it "surprisingly difficult to hear any difference." Later he said he felt the ML was a bit better. He also compared it favorably with the Musical Fidelity TriVista player ($6000). In giving the B. the 2004 Stereophile Editor's Choice Award, JA said he was "floored" by the B. and compared it favorably with the ML 30.6, Theta Gen VIII and Wadia 27ix, saying only that the DAC1 didn't sound "quite" as "expansive" or "smooth", but that its measured performance was "as least as good as, if not better than, the best..."
A few days ago on Audioasylum, John Marks of S'phile mentioned it as essentially equivalent to dacs costing $5,000 and $10,000 (except in the price, of course, which is what makes the DAC! so unusual).
The many pro and audiophile reviews and posts overwhelmingly support the above. I have posted this information since, as an owner for 4 months, I am constantly amazed at the DAC1. I never thought I would have world class digital equivalent to ML, etc. in my system in my lifetime, but now I do! I feel I owe Benchmark a vote of thanks.