Counter-intuitively, it's the large speakers that often don't need as much power to reach a high SPL. Many bookshelf speakers have a sensitivity rating in the 83-86dB range. A 25 wpc receiver could only drive a speaker with 83dB efficiency to about 98 dB. OTOH, a humongous Klipschorn or La Scala hits 105 dB on 1w input. 25w would drive them to about 119 dB, which is LOUD! As for modern designs, some of the 6- and 7-foot tall line arrays have sensitivity ratings in the mid-90s.
Bob Carver designed the original Phase Linear 700 (350 wpc) with the dominating "bookshelf" speaker of the day, the AR 3a, which had a sensitivity rating of 83 dB. Bob knew a speaker like that needed 200-350 wpc to come alive.
Just about any small bookshelf speaker with good bass response achieves it by padding down the volume of the upper range so the bass can be heard. Bookshelf speakers, especially acoustic suspension designs, are notoriously inefficient. The other things said about the vintage Marantz receivers are true. They are pretty well built, have good current delivery, and have good oomph at whatever their power ratings are.
And not all smaller speakers are inefficient; but at 25wpc the more efficient the speaker, the more the music will come alive.