Not a good idea... those open back cabinets are designed as modified infinite baffle enclosures (infinite baffle blocks the rear sound wave from mixing with the front sound waves, causing cancellation at various frequencies- a TRUE infinite baffle is (duh) huge, so a smaller version with folded sides is usually adequate to prevent mixing of the front and rear sound waves.

On the other hand, you can run the amp output thru a closed, or properly ported, cabinet. A 12 or 2x12 or 4x 12 with the proper impedance can make a big difference in sound.

Seimic Audio has some good stuff!

rock on

Sounds like a headphone connector gone bad- when you plug in the HPs, a lot happens! IF one of the leaf switchs inside the 1/4" connector got bent, you sometimes have a feed-back loop just waiting to go crazy.

Try the amp with some headphones plugged in- it might help lead you to an answer.

I didn't find the exact schematic but Schematic Heaven Dot Com has the VS230R, which might help.

http://schematicheaven.com/marshallamps/vs230r_2x30w_stereochorusrev.pdf

Whenever I want to reduce my amp to half power, I just move the VOLUME knob from 10 to 5. It works for me.

Seriously- If you have a 2 speaker setup, make up a switch to change the speaker hookup from parallel to series. Be SURE to use a NON-SHORTING switch or ALWAYS remember to turn off the power before you change the setting.

If you'd like to have a go before messing around with wires and such, try it out by using a jumper cable. Carefully pull off the speaker connectors (remembering where they were originally connected!!) and re-connect one output lead to one speaker and the other output lead to the other speaker- hook the 2 speakers together with a jumper between the 2 open connectors... Simple! You go from 8 to 16 ohms, no worries!

Be aware of speaker phasing- you probably want to keep red to black (or + to -) but you might prefer an out-of-phase sound! Try both ways and listen!