i tested a lot of different opamp combinations with the line outs the last weeks (I'm using the smk-iii as a dac). The opamps i tested were the op 275, opa2604, opa2134, ad823, and ad8066.
First of all, the hlly smk-iii (mod) already provides a good sound out of the box and is quite worth its money ... However, replacing the opamps improves the sound quality noticeably:
Out of the box, the smk-iii is equipped with op 275 as filters (from the front, the two opamps on the left side) and I/Vs, which provids a lively and moderate detailed but also a somewhat un-harmonic, bright, "electrostatic" and aggressive sound (especially the bass, wich is quite interesting for the first hours but becomes annoying as time passes); completely lacking a "soundstage" in comparison to the following combinations.
Here are the highlights:
filter: op 275 & I/V: opa2134: The opa2134s make the sound a little bit darker and laid back and also broaden the soundstage.
filter: op 275 & I/V: ad 823: The sound is somewhat bright (but not too bright) and has a "electrostatic" quality, gains a wide soundstage and is much more detailed. The bass is very punchy. In all, the sound is lively but the bass feels a little bit out of place. However, a very enjoyable but analytic combination. If you like electronic music, this might be your choice.
filter: opa2134 & I/V: ad 823: Same as before but less aggressiv and less refined (the bass is less punchy), somewhat laid back, and a bit darker (just a bit). Seems to be better for long listening sessions than the previous combination (might be a bit less detailed, but almost not worth to be mentioned).
filter: ad 823 & I/V: ad 8066: Excellent soundstage and detail grade, the bass is punchy but not as dominant as with the op275, the sound feels somewhat "electrostatic" and is lively but also a bit bright and "un-harmonic". Gets the second place award.
filter: ad 8066 & I/V: ad 8066: WOW! My absolute favorit! Excellent detail grade, very lively and harmonic sound and an unbelievable broad soundstage (also the best locatability) but not the typical burr brown "laid back" or op275/ad823 "electrostatic" sound. Some songs gave me the impression the singer stands in front of me playing his guitar (jack johnson) and others let me believe the singer stands 10 meters away framed by a orchestra (celine dion). The ad 8066s provide excellent mids and highs (really impressiv), not bright but also not dark, just perfect. The sound is very harmonic neither the highs nor the bass, nor mids feel out of place. A very very easy "to-listen-to" combination. The only deficit might be the bass: The bass is goes deeper than with the previous combinations and feels therefor less "tangible" and punchy (in fact it isn't - it is still as good as with the others). I first switched back to the previous mentioned combination because of this - i thought using the ad 8066s as filters would lack bass. This is definitely not the case. The bass is still very good but is more in the background and not as dominant as with the previous mentioned combinations, which, in all, allows for the great harmonic sound. You should go with this combination if you have speakers/headphones that deliver a deep bass (not necessary a dark sound). Otherwise i recommend one of the previous combinations. Also a pa2227 & ad 8066 combination should be worth a try...(let me know if you did try it;-)
With the last two mentioned combinations the hlly smk-III (mod) as a dac provides an excellent sound!