The term "piano quintet" actually designates a variety of five-person ensemble combinations. As mentioned earlier, by far the most standard group involves piano + string quartet or piano-violin-violin-viola-cello. However, other groupings are possible: piano-violin-viola-viola-cello (Paul Juon), piano-violin-viola-cello-cello (Henri-Jean Rigel), piano-oboe-violin-viola-cello (Théodore Dubois), piano-flute-clarinet-bassoon-horn (Nikolai Rimsky-Korsakov), piano-clarinet-horn-violin-cello (Zdeněk Fibich), piano-oboe-clarinet-bassoon-horn (Beethoven), etc.
The standard ensemble of piano + string quartet has the advantage of matching the piano's wide range with four instruments that cover the same tonal space... almost. Truth be told, the piano actually goes more than an octave lower than the lowest note on the cello. A handful of composers have apparently felt that the strings should match the piano in the lower regions, and so have written piano quintets for piano-violin-viola-cello-double bass.
Herman Goetz (1840-1876)... I'm drawing a blank on some sort of beard-related joke. Come up with your own!
Herman Goetz was a German-Swiss composer who wrote such a quintet. Take a listen to the opening of the first movement. Throughout the sombre, moiling introduction, you may be able to detect the double bass grumbling along down below. Then, at 1:34, the mood changes... (I'd suggest listening at least until 3:41, when we hit the repeat sign.)
The drama promised and forewarned in the slow introduction, bursts from its proverbial dam and flows along "Allegro con fuoco", lively and with fire! Weeeeeee! In those downward, scalar gestures that begin the section, you can really hear the double bass go. To my ears it adds serious "heft", a word I use for the sense of weight being heaved about, but also for the sort of raspy, onomatopoeic quality you hear when the double bass bow digs into the strings. This instrument reminds you that bowed string instruments make their sound by rubbing or scraping wires, hairs, guts, or other strings perpendicularly against each other, a fact that the melodiousness of the violin, viola, and even cello seem to mask.
One more example: here's the last movement, a sprightly yet still hefty sort of dance. The double bass has some great moments, adding some weight to the section marked "pesante" or ponderous at 0:37 (which is an interesting juxtaposition of weightiness and dancing rhythms), doubling the cello at the octave in the fugal section (starting 1:33, bass in at 1:41ish), the weird, trembling fade out at 2:55, and of course, the killer dash to the ending starting at 4:40.
Check out the other movements as well! Have a grounded, bass-heavy day!