Sibling rivalry.
To remind ourselves, here is a look at the Job 225.

This helps us appreciate how very similar the 250's circuitry is. Except for some color changes on certain capacitor housings and some omitted parts, we're essentially looking at the same amp converted for mono operation by applying what served two channels to one, with a fatter taller transformer and more heat sink surface thrown into the bargain.

Here's a final angled view on the board and octet of output devices. Like the Job 225, the 250 isn't a DC enlarger but pass-alonger because it's direct-coupled. That means you don't want to present DC to its input since that'll be forwarded to your speakers as noise and possible tweeter conniptions. This could eliminate lesser tube preamps in particular, not that I ever had any issues with my Nagra Jazz or equivalent tube kit from Octave & Co. But such combinations are obviously not likely given the Job stickers. If you do plan to mate these babes to an affordable valve preamp, make sure it'll work flawlessly before you commit. As high-gain circuits, the monos will amplify incoming noise. If your valve preamp is microphonic or otherwise noisy—most of the lesser ones are— don't point a finger at the 250. They merely magnify what they're fed 56 times. End of small print.