I would do a search in the amp and cab forum first, and see if any of your 2 items have sold here in the past.
Then go to Ebay and do the same. Make sure you set the Ebay serach criteria for UK only, Used, and the hit the SOLD ITEMS.
If you see any past sales on here or Ebay, disregard the very cheapest SOLD items, as this will eliminate a desperate sale, as this isn't a true
indication of value, and then disregard the over the top prices ( if any ) as some people on Ebay get Bid Fever and pay over the odds.
Or simply take all the prices and do an average and see what that is