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Do I have to alter or remove the R6 resistor (bandwidth limit) on the MH board when changing from buf634 to dDB ?

No, there is nothing to be changed or removed when moving from buf634 to dDB.

Do I have to change the output resistor (R4) ?

No, this resistor doesn't has to be changed, either. Since the dDB doesn't incorporate a current limiter or overheating protection you may consider to populate R4 with a 10 to 20Ω resistor if you jumpered across R4 with buf634 / opa551. This protects the output resistors from overheating and getting fried in case you permanently short the output (e.g. by plugging in a headphone plug halfway).

Is the dDB compatible with opa551 mod / second board revision ?

Yes, the trace has no effect on the dDB. The dDB is compatible with all revisions of the revised Millett Hybrid.

Is the dDB a direct drop in replacement fur buf634 / opa551 ?

Yes. There is nothing that needs to be changed in any way. The dDB connects to the pins IN, OUT, V+ and V- only, all other pins of the dip-8 socket on the rev.MH aren't connected to the dDB and therefore have no effect. Simply take the buf634 or opa551 off the board and plug the dDB board in. That's it.

Has the dDB been developed particulary for low impedance headphones ?

Not particulary, high impedance headphones like the Sennheiser HD650 or Beyerdynamics DT-880 benefit as much as low impedance headphones like Grados. The dDB omits extra circuitry like output current limiting, overheating protection or bandwidth limiting that are incorporated in the monolithic buf634. Though buf634 is a diamond buffer, too, the discrete implementation takes sound a step further ahead.

Can the dDB be used in other amp designs?

Generally speaking, yes. Nathan has successfully implemented them unmodified into his PIMETA. Please note that depending on the amp's design the dDB may not work at all or may need some midifications (e.g. isolate the output from the feedback loop by a resistor). If someone tried the dDB in other amps I'd be very interested to hear about the results.

Does one DB board include both channels? Seems like the ones I've seen straddled L-R buffer sockets.

Yes, the dDB is a stereo board and features 2 channels on one pcb. The straddled one channel boards were prototypes.

How much did the typical group buy board cost?

Round about $5

How much does it cost to populate the board?

Approximately $25