Lowboy, I have basically given up on trying to find one amplifier that will work with all melodicas. I have found that you need to round out the type of sounds you are looking for as well as rounding out the melodicas to be used together. Example
In my case
Fender Acoustasonic Junior = Vibrandoneon * Eolina * Hammond Hyper, *= using a Myers pickup
Fender Delux Reverb = Hammond 44, Hammond Soprano, Hohner Electra
Magnatone 421 = Hohner Solist, Hohner professional, Hohner piano 36 using a Myers pickup
Traynor K4 = All Hammond Melodions including Bass Melodion, All Hohner melodicas listed above except piano 36 and Basso,
The above are my most used melodicas. The Clavietta seams only to work with a Microphone so I can only use it with the Fender Acostasonic and Traynor K4 of which I am not at all satisfied and usually substitute the clavietta with the Hammond Hyper.
So you can see a basic pattern on the typ of related sounds of melodicas the work with the type of amplifier used. All the applications above are affected substantially with the use of effect pedals.
I am very interested on your signal path from melodica to amplifier and actual tone settings. If you are using a microphone on your Vox this is what I am most interested. I have tried many different set ups with DI boxes that have not given me what I am looking for when using a microphone in a “live performance” application.