If you want to buy something without upgrading later on I can vouch for the GAP 73, a very nice usable piece.
But the poster did say tubes so I would say a hidden sleeper is the older TLA PA1 or PA2 mic pre valve unit made in England models. Absolutely othing wrong with these and pretty cheap as well.
Another heavier coloured unit would be the ZOD IDDI unit. That has a fair amount of gain and it can also be used as a mic pre if you have a hot enough LDC mic. Cheers