This is from an old release by Bryston on the ST series:
"PROFESSIONAL versus REGULAR Power Amplifier Models
There are three differences between Bryston’s Professional series power amplifiers and
our Regular models:
1) PRO models have a large “Bryston†logo on the front dress panel to make
identification easy. REG models have a smaller logo in the upper left quadrant.
2) PRO models have individual gain (input level) controls for each channel. On 2B
through 8B models these gain controls are mounted on the front. On 9B and 14B
models they are mounted on the rear. Most front mounted gain controls (except
the 2B-LP) do not have knobs, but instead use slotted end metal shafts slightly
recessed behind the panel to prevent accidental adjustment. These controls can
be adjusted with a small (3/32†wide) blade screw-driver. Most rear mounted
gain controls, such as those found on the 9B and 14B, are equipped with knobs.
Except on the 2B-LP, whose gain control functions like a conventional audio
volume control, these PRO model gain controls provide a restricted attenuation
range of approximately –14dB. However, any PRO amp can be ordered with a
full range gain control at no additional cost.
3) Some PRO models provide different inherent gain and/or different gain options
from REG models. This is due to the fact that a balanced input inherently
provides 6dB of gain if each of the two halves of the input (or if each input phase)
is operating at unity gain. This 6dB of gain is sometimes desired by professional
installers, but can be an annoyance in home installations where every successive
balanced in/out stage in every piece of equipment in the audio chain adds 6dB of
gain. This can quickly add up to an excessive level. In general, REG models do
not usually add 6dB of gain at the balanced inputs, whereas PRO models do.
All Bryston power amplifiers are now made with an overall unbalanced
gain of 28.3 (or approximately 29dB) and are thus compatible with THX levels.
PRO versions of the following models are available:
2B-LP, 3BST, 4BST, 5BST, 7BST, 8BST, 9BST, 14BST
"
