I'm looking to get a vox vt 20+ modeling amp i noticed that there are a few effects, and one that i didn't understand was clean v.s. cali clean. whats the difference?
STD (Standard)
This models the clean channel of a high-quality amp that was produced only
on special order, and was known as the overdrive special. With a beautifully
rounded low range, a sharp midrange attack, and a sweet treble register, this is
ideal for single coil pickups.
SPL (Special)
This models the clean channel of a Japanese-made amp with 2 x 12" speakers
that went on sale in 1975. It is known for its clean full-range sound and its
built-in stereo chorus, and is used on stages and in studios around the world.
CST (Custom)
This models is only a three-band tone control amplifier that produces a pure,
clean tone. Setting TREBLE, MIDDLE, and BASS to the center will produce flat
response of the pre amp.
NOTE: If the input level is too high the sound may become distorted. To
prevent this from happening reduce the gain in the preamp.

STD (Standard)
The 6G5-A “Pro” amp was produced during the years 1960–1963, and was
distinctive for its yellowish brown vinyl cover and round brown knobs. This
40W combo amp is known for its warm and clean tone.
SPL (Special)
This American-made tweed-covered 2x12" combo amp made in 1957 is known for
its rich and clean tone that’s ideal for classic rock, blues, and country. By raising
the volume you can also produce a powerful and punchy overdrive sound.
CST (Custom)
This models an American-made black-paneled amp that has been modified.
With this modification, an already-superb amp gains even greater smoothness
and additional warmth.

From the official users manual.