well, instead of modding the HT1, which I really like, I decided to try a HTV-112 cabinet.
Initially I was in doubt if this is not too much (Celestion 80 with 16 Ohm and 80 Watt) for the little screamer, but boy, it does the trick. Really loud (I cannot believe it is just "One Watt") with a nice basssss and it fills the room with music thanks to the open back. 5 stars.
Buying a new cabinet is maybe more expensive than replacing the internal loudspeaker, re-fumble the tubes, etc. But I am quite shure, it has a much bigger impact on the sound experience you get as a result... and you can use a 12 Inch cabinet with anything but a toaster.