If you already had a HD350, why bother 'upgrading' to what is essentially being marketed as 'the same amp, but ten watts louder'?. Ten watts is almost a negligible increase, so negligible that I am surprised that EBS even bothered doing it.
I have tried the HD350, 650 and Fafner. The 350 was my least favourite of the three, although still a very nice amp.
Getting a poor sound at the venue could be down to a number of factors. Unless you could simply switch the HD350 back in and see if that made a difference, it could be down to monitoring or changes in levels from any one of the band.