1978 Marshall JMP MK2 Combo

I recently aquired a vintage 50 watt MK2 Super Lead 2 x 12 combo. It was one of the first Marshalls to feature a master volume and pre amp control. A predecessor to the JCM 800 I think. Was wondering what it might be worth. I practically stole it at a yardsale. The grill cloth is ripped and the front cab corners are missing, but the Marshall logo is intact. Speakers are "Marshall" but are probably made by Celestion. I have retubed it and re-biased it. Runs 2 6550's for power. A plethora of tones are lurking in this amp-shimmering cleans to AC/DC. I put a Marshall GUV'nor 2 pedal in front for heavier stuff. Also it has two extension speaker jacks but no ohm information. I have a CRATE 4 x 12 that runs at 4 ohms. Can I use this amp to push it without damaging it? :confused: Any information would be most appreciated. Peace