As far as a i know, there are at least five different ST-140's that B&K have produced. That is, they have the first model that was rated at 70 wpc with an "old school" iron core transformer. From what i've been told, this unit was quite limited in production. They then came out with the second version, which was rated at 105 wpc using a toroidal transformer. Sometime during the 105 wpc run with the original Hitachi devices, they changed quite a bit of the internal design / lay-out of the amp. One of the signs of the latter "third series" design is that they went to a "star ground" i.e. all of the ground wires tied to one point in the center of the chassis. According to what i've been able to find out, these design changes came into play appr 4 years or so after these amps were initially introduced, producing the third version of this amp. As such, the earliest versions PRIOR TO these changes are the most desirable in terms of sonics. The fourth version was this same amp but configured into a mono-block. They then switched output devices as the original Hitachi Mosfet's that they used were discontinued, resulting in the fifth version.
Having said that, it should be noted that the original Hitachi's are no longer available. If you end up blowing the output devices, you have two choices. You can send the amp back to B&K and they will rebuild it to current production standards for a very reasonable fee. They make use of the existing chassis, transformer, rectifiers and filter caps while replacing the amplifier circuitry and output devices. In effect, you end up with one of their current amps for about 40% of the cost.
The other alternative is to substitute some English made Mosfet's that are a direct replacement. This same company also manufacturers heavier duty versions of this device. It is essentially two Mosfet's in one case. Some fine tuning of the circuitry may be required, but this would result in a far more rugged amp.
As a side note, MANY, MANY other amps used these same Hitachi output devices. Earlier Perreaux's, Muse, etc... along with quite a few others used these Hitachi's, so there were tons of them on the market at one point in time. It has now gotten to the point of searching for "hen's teeth", so be careful with what you've got. That is, if you like the way that it sounds as it is. Changing the output devices and / or support circuitry will surely alter the sonics of the unit. Sean