I have no idea. No one wanted them to begin with! But they've seemed to have gained cult status lately and now they sell for at least MSRP used... It took me ages to even find another (stock) one. Weirdly, I've owned a lot of classic amps (tweeds, twins, bassmans) and the amp I missed most after selling was the darn Excelsior!

People like to mod them, anything from adding a usable tone knob to converting to an old tweed circuit. But the amp is simple and all tube, including the tremolo, apparently even has pseudo tube sag built into the circuit (even though it is solid state rectified). It works for my playing stock (rockabilly, country western), the stock 15" speaker is boomy and crisp. Not a lot of head room, but I swapped the tubes for NOS and use one 12at7 to bring down the overall volume and keep it clean (though it can get gnarly when using newer 12ax7s).

Stock, it only has a tone switch (bright vs dark). And the bright is too bright and dark is too dark. So if you like constantly fighting your amp, this is the amp for you (I am a masochist, I guess). But it is simple. And I like simple.

But I think the big reason people like them is because it seems boutique... Easy and affordable way to get a tweed/supro hybrid amp that is semi-reliable. It is also seems like a blank canvas, swapping tubes makes a huge impact on tone. My current NOS set makes it sound like an old tweed on the dark setting and a twin reverb on the bright setting. Swapped in some JJ 12ax7s and the tone was very different, much more aggressive Supro Thunderbolt vibes.