For Valves 10w was actually a high power in 1958 so 10w minimum unless an early Integrated.
10w in Valve terms relates to about 35w in Transistor volume, from comparisons.
Interesting to see how often these were used & why modern amps still use them.
There are other output valves noted, but of less interest or not used today as often if at all.
There are 2 early integrated amps & that bad-idea tri-amping that got revived later on.
Some of these will now be unfindable & £1k sort of items.
The first book attempts to be an all-in guide to Hifi covering a lot in one 180 page book.
They will be very old by now & 25w valves will sound nothing like their best or what you'd like perhaps.
They will be very musical, but bass light & treble rolled off.
A true Retro sound like a big 1950s Jukebox, fun factor high, fidelity soft.
Not to buy & try to upgrade, to get working to the old spec is the wise thing on these as buyers won't like you hacking holes for Phono Plugs & 4mm speaker sockets if there are none.
As the years go on, this eliteness fades as readership will have been small.