I mean if you need the attenuation to get the sound you want at a decent level then you should get one. The Morgan MV is pretty transparent though, and if I'm not mistaken (though I probably am) don't Morgans come with very nice power scaling?
^some do, some don't but I believe it's an option you can ask for. Depending on how long you have to wait you might want to try out a Badcat Unleashed. It may be way more than you're looking for, but having the option to be louder than your amp can actually be, as well as quieter makes it useful imo.
I don't believe you will need an attenuator. It has a good master. It is very different than any master volume I have ever had though. The Volume and Output knobs are very interactive (as well as the tone knobs). You need to experiment and know that slight tweaks of the knobs can make a big difference. You can get the amp to drive by either opening up the Output (master), even when the volume is low, or by doing the opposite. Experiment and find the sweet spot. I don't think you'll need to attenuate. If you do, get it after you try it.