Quatre dg-250 amplifier

The early gain cells were best with 8 ohm speakers in terms of stability. If one blows up you may have hard time sourcing transistors. Check this link:

http://www.diyaudio.com/forums/showthread/t-4255.html

I have owned 4 gain cells over the years.

The GC 300 was the best sounding model and the best capable of driving speakers with difficult impedance properties. The next best was the DG250C. Quatre/QMI licensed the 250/300 designs and produced variations and made changes the designer did not sanction. In particular, the 500 I heard sounded terrible and these were prone to blow up.

Mechanical hum may be caused by the transformer mounts - grommets broke or bolts loosened up.

Some 300s were retrofitted into 250 chassis. These are more desirable than the flimsy chassis QMI used on the factory manufactured models.