Active monitors feedback problem

[Matt77]

My home set up is as follows:

 guitar -> Amp -> SM57 -> Boss digital recorder (maybe a touch of chorus, reverb or doubling) -> Alesis near field monitors = Stadium Rock at low volume

Essentially it's a Dry / Wet rig.

I can get a great sound from close miking the back of the cab
The problem is when I turn up the monitors I get feedback .Bad feedback sounding like the horn on a transatlantic ship.

I've been trying to negate this once and for all today by moving the speakers and cab around etc.
I've been wandering around the place mic in hand to isolate the problem. I stripped everything back so that I had just 1 active monitor on and the mic. No guitar or cab plugged in. I can walk around the room with the monitors up full and the mic input up full without any problem. As soon as I put the mic near the cab the feeback begins even with the cab totally unplugged. I think it's the magnet in the celestion that causes this as it's worse if I put the mic at the back closer to where the magnet is. If I take the cab out of the room and hold the mic near it with the door shut it is fine, but open the door an inch and it feedsback again.

space prohibits me from keeping the cab and amp set up in another room
I'm thinking of building a coffin for the cab to isolate this and cure the problem.

Is there an easier way?

Any suggestions would be welcome

[Ratrod]

Do you really need a monitor setup at home?

In a confined space it's easy to get feedback. The mic picks up the monitor.

[Matt77]

The answer is no! But I don't need so many guitars either :D
I can use the monitors to add effects and it sounds great playing along with CD's, loops and backing tracks that I have. I'm just greedy!

[sgmypod]

how far away is amp and mic from monitors

[Matt77]

it's close but it doesn't seem to make much difference. I've tried every bit of the room by walking around with the cab and mic.
It's just when it gets close to the magnet in the cab.

[indysmith]

What polar pattern is the microphone you're using set to?

Maybe I've missed something, but if you're recording something and monitoring it straight back into the room containing the microphone, the only possible outcome is feedback unless you're extremely clever with the microphone placement.

Maybe use some amp emulating software instead, and that way yu'll have all the effects you like and no microphone and feedback?