A solid-state rectifier feeds the B+ rail through silicon diodes, whose forward resistance is near zero. The rail barely moves under load, so there is essentially no sag — the opposite of a tube rectifier, whose voltage gives under a hard transient.
Builders went solid-state for headroom, stiffness, and reliability. With the rail held firm the output stage delivers its full voltage on every transient, so the amp hits harder and stays tight where a tube rectifier would compress; the diodes are also cheaper, cooler, and don't wear out. On the British lead amps that firmness — fast attack, no give — is a defining part of the voice.
The trade is exactly the feel a tube rectifier is kept for: the dynamic compression and bloom of a sagging rail are gone, replaced by immediacy. That contrast is why the archive documents both.