You have said thank you and such...I have no problem with your posts...I am just giving you my own opinion based on what it sounds like you want to do...I tend to be pretty straight to the point so I am sorry if I seem terse...I don't care whether you use these suggestions or not...That's all they are...for the pickings...Along with all the other information you get from other folks here and your own outside sources...

I strongly suggest you research and learn as much as you can before dropping $300 on a THD box...But the bottom line on any gear decision like this is that you like what you hear. That's really hard to quantify for something you can't tryout first...The desicion between a 16 ohm and 4 ohm less to do with what THD says IMO...Did you know that there is a much higher risk for flyback voltage running a Class AB Tube amp on the 16 ohm tap vs. 8 ohm and 4 ohm is even safer...It is high impedance or a no load (infinite impedance) condition that causes increased flyback voltage on the output transformer and it is too much flyback that blows output trannys 99.9% of the time...Class AB Tube amps tend to get a little darker as the impedance goes down...Brighter and thinner as it goes up...It's very subtle, but might be an important factor in your tone search...

My own opinion of that THD box is that it is extremely overpriced for what it truly is. If you just wanted to cut your power in half you can get a 4 ohm dummy load for about $20 or even less if you look hard and run that in series with your cab in 4 ohm mode. That would make it an 8 ohm load and half the power soaked out...Add another 8 ohm dummy load in parallel to that and you are back to 4 ohms with 75% of the power soaked out...Add a rotary switch and you have the same thing....You can DIY a hotplate for about $100 for a one off...Of course the fancy billet box adds quite a bit to the cost...The THDs work, but the way they soak power is not the same as speaker motor...A guitar amp is designed for a reactive load...A static load changes the tone...That's subjective...I don't like the way dummy loads affect the tone so I don't use them...

Here's a different approach for you on knocking the power down and the cost is about the same as a THD...Groove Tube Power Reducers...These are EL84 Power Tubes with an adapter for the 6L6 socket...This turns the amp to Class A and knocks it down to about a 30 watt amp...You even have the option on a 4 tube head to run 2 PRs and 2 6L6 5881...Basically that is exactly the same as a simulcalss Mesa Mark series...There is another brand called Yellow Jackets that are just a tad bit cheaper...My brother has a pair of GT PRs in his old Ampeg 60 watt combo and it took on a very nice Vibe...One advantage to these is that the amp becomes Class A and you do not have to bias the amp...If you have your bias set for your 6L6 tubes then you can swap them in and out as needed and the mixed mode is very interesting...