The 1704s are legendary and in the configuration Lenehan uses, supports only inputs of up to 24/96.
True when it comes to "legendary", but masking their real colors with a tube stage does not really improve their performance (that I personally find unsatisfactory). You need to hook up the MPD3 internal DAC to your tube output stage in order to find out how the two DAC designs really compare.
Best wishes,
Alex Peychev
APL Hi-FI