Hi

The power amp stages are to all intents and purposes identical

The DSP/Pre amp section is where all the main differences lie:

The MkI has a reasonably large number of factory presets (about 300 in all at a rough estimate) and factory user presets that you might find useful, however you cannot connect the amp to a computer to: update the flash memory; edit presets on a computer; store presets on a computer.  The MkI can only be edited from the front panel.  FX are limited to the few that are accesible from the front panel.

The MkII has less preset slots overall (128) but ALL are user editable.  The supplied factory presets total 64.  The Spider Valve MkII can be connected to a computer by MIDI or by using an FBV MkII Shortboard or FBV MkII Express as the interface.  The MkII can be updated and since its inception it has been and new FX models have been added in two major updates.  There is a computer based editor/librarian program called Spider Valve MkII Edit.  The amp models have the same names in the MkII as they do in the older MkI but they have been improved.  The Spider Valve MkII can be controlled by another device using MIDI and the Spider Valve MkII can control other devices by MIDI.

Both Spider Valve MkI and Spider Valve MkII 40w combo amps are very loud for their power rating.  The Spider Valve MkI and MkII HD100w heads require a speaker cabinet capable of handling 100w.   The head is also capable of making a loud noise through an appropriate 4x12 or other format speaker cabinet.

Both MkI and MkII amps have a serial FX loop (Pre-Amp Out and Power Amp In sockets).   Both MkI and MkII have an XLR Direct Out socket at microphone level which can be used to supply a DI signal to a PA desk.  Both amps can send a signal to the PA in Standby mode which is good for late night 'silent' recording at home.  The MkII amps have a Live mode wjith speaker cab emulation which takes the DI output signal directly from the output transformer after the main power tubes (you must have a speaker load connected and the Standby switch must be ON for this to work - plus you have to configure the amp for Live mode using the editing functions on the front panel).  The MkI does not have this.

There is one main issue with the MkII amps that some users have a problem with, and that is the switching between patches can produce a 'pop' sound usually if the patches have been saved with the channel volume set lower than about 50%.   Both MkI and MkII models have a sensitive Master Volume potentiometer - particularly at low volume levels and it can go from silent to pretty loud quite quickly - and this is the main reason some users store their presets with the channel volume set lower than 50%.

The MkI is discontinued, whilst the MkII is the current model range.  Both amp ranges work well as a host power amplifier for a modelling solution like any of the POD HD range.

I don't think I've left anything major out

Nick