I've removed my post entitled "DSP interpolation for the masses" because it was partly based on the misconception that DACs do not actually attempt to implement digital filtering internally. That misconception was based on misreading an interview with Ayre Acoustics' chief designer Charles Hansen, who was commenting on an "interpolation" process which takes place AFTER the interpolation performed by the digital filter.
However, it turns out that, according to Hansen, the digital filtering performed in DAC chips is performed in stages, and that round-off errors accumulate. External filters use a "one pass" approach so that round-off errors occur only once. Perhaps it is this reduction in round-off errors which provides the dramatic improvement I've noticed with DSP-based filtering. (Even Wolfson's high-end DAC chips have inputs for external filters.) I'll continue to study the issue.