I up-dated the firmware in my 861 from 4.2 to 4.24 and noticed a clear lack of bass.

Meridian recognised the error in 4.24 and quickly released 4.25; I up-dated the firmware again but this did not fix my problem.

Contacted Meridian technical support (St. Edd) today and Edd recommended the following:-

Re-type the 861 to any of the default types (I used type 0).
Make a new configuration file from scratch (not easy with an 861).
Store the file.
Calibrate the system.
Carry out room correction.

Perfect; the above fixed the problem.

Bass is back and the sound is fuller, in fact I would say the overall sound is improved.

A big thank you to St. Edd.

Meridian 800 DVD V4, DSP7200.2's