I put one of these in a head cab, and I go along with everything James says. But for two speakers, you don't have to have a connecting jack in one of the cabs. Just get a Y connector to plug into the amp's speaker jack, and run two speaker cords from there - that makes parallel circuits.
Separate speaker cabinets are already pretty light, so you don't necessarily need neodymium extralight speakers. I have JBL D130s in Marrs cabinets (Duane Marrs), and they weigh only 23 lbs. each. I also have a single Eminence 15" in a large cabinet (48 lbs.), but the JBLs sound much better to me. Generally, two light or medium duty speakers will sound better than one heavy duty speaker. Even though the JBLs are rated at only about 60 watts each, running two gives a rating of 120 watts. Also, the 60 watts is an open back rating, and I use closed backs, which are much safer. In addition, I use a volume pedal and don't attack notes with it all the way on. So my JBLs are safe with the way I use the 180 watt STR.
The dimensions you will get for speaker cab design will invariably be for PA woofers or bass guitar. You don't really need a cab that big for steel, because you don't have any notes down where a woofer cab is designed to resonate. The smaller, more convenient cabs you can get from Marrs and Rick Johnson are fine for steel.
I prefer closed back cabs, because they prevent speaker blowout, are more efficient, have better lows, and have a more consistent sound in various venues. The only time I would use an open back, would be when I have to place my speakers in front of the drummer - he would have difficulty hearing closed back speakers, because they project all the sound out the front. But some people like the open back sound. You can get cabs with a removable or sliding back door, so you can use them open or closed as needed.
As for the STR, you should be aware that the default setting without a boost footswitch is the no-boost setting. It will only have about the volume of regular Twin. To get the full use of all that extra clean headroom, you need have a switch plugged into the boost jack and switched on.
Also, the default setting for the 5-band active EQ is for it to be on along with the regular tone knobs. This means all the knobs work at the same time and interact. You can get a very complicated EQ setting that way. But the real value of the two separate EQ systems is to have the 5-band setting switchable. That way you can set the regular tone knobs for one setting (say brighter) with the 5-bands off, and hit the switch to get another setting (say darker) by overlaying the 5-bands over the regular tone knob setting.
Good luck with the STR. You have a huge amount of clean headroom and Fender clean tube TONE. And you wont need heat in your practice room.
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<font size="1">Student of the Steel: Zum uni, Fender tube amps, squareneck and roundneck resos, tenor sax, keyboards
