The universal 334 has the most realistic timbre for string instruments i've heard to-date. clearly better than my impression of JH13, Heir 8a or any of the demo customs i've heard. There seems to be a hint or presence of bass that may pounce out across the spectrum, reminding me of the Heir 8a. Decay has a reverb that lingers a second or so, very nice. Am sure the custom version will outdo the already impressive Heir 8a. Rubick beware ...
The custom 435 was quite the surprise. There is a color to its sound yet sounding very natural and dare i say, realism to the vocals. The mids are ridiculously the best i've heard of any custom, universal or headphone as well. The sound signature feels like you're very thirsty and haven't drank for days, suddenly drinking clear fresh water feeling. That fresh mint like feeling. The bass weight seems to be a bit on the light side (and this is coming from a JH13 user), and may be a dealbreaker for bassheads, but the rest do it so well, for me it more than compensates.
Outrageous, though just calling as i hear it. Will probably need another round or so to get a better grip of its sound signature.
First impressions, id say the 334 and 435 are very complimentary. They have strengths that make them amazing at their respective sound signatures. Together, i feel like i'm not missing anything and could well be the end game combo (though a very high priced one) at least to my ears.