Thanks for the tip. I'm using a Fender Twin output transformer and I found out my schematic was also backwards. I went back and re-drew it so that the side of the phase inverter that is NOT inverted goes to the 6L6s that are connected to the same 'end' (schematic-wise) of the output transformer as the 'ground' speaker lead. I am still waiting on my kit, so I didn't have the opportunity to hear it HOWL like you did.

So, on my schematic, the non-inverted signal from the phase inverter (pin 6) goes to the output tubes that connect to the BROWN wire of the transformer.