Is weight really mater?

John, in general the answer is no, weight is not a factor in amplifier performance. High performance can be achieved in amplifiers that weigh a lot, and in amplifiers that weigh relatively little. In dividual designers make certain technological choices that end up leading down particular technologies, topologies. . . and weights. In the end, there are excellent amplifiers at all weights, and horrible amplifiers at all weights.
In the particular case of the JRDG 102, the designer wanted to achieve a high degree of performance for a chosen constraint. . . it was not weight. . . but an entry level target price of $1500. He used class D amplification and regulated switch mode power supplies that are extremely energy efficient, hence the very low weight. Whether Rowland achieved his price/performance goal on this particular product or not can be decided only by using one's ears. Unfortunately a scale won't help.
Will JRDG 102 outperform a Levinson 331? Perhaps possibly, but I truly doubt it. The 331 was designed with more ambitious goals. and it is priced 3 times higher than JRDG 102. While price alone is not a particularly good gage of performance, it is somewhat a better indication than weight.

On the other hand, if your budget can go as high as the $2K for the 331 currently offered on Audiogon, I suggest you also have a look at some of the bel canto Ref 1000 monoblocks available on Agon. I have reviewed one of these little wonders. . . and they are quite wonderful. . . and yes, very powerful as well. .. . and won't turn your living room into an oven. . . 331 runs in class A, which means it dissipates a huge amount of heat. . . not great for Houston Summers. G.

Ngjockey - Efficiency of class D plays a role but the most important difference is Switching Power Supply. Toroid in switching supply has diameter of 1-2" and can carry as much power at 50kHz as huge 10" dia toroid at 60Hz. In addition linear power supply is not regulated and requires big amount of capacitors to hold the voltage at loud passages (and filter out 120Hz) while SMPS is regulated (line and load)and easy to filter (small capacitors).

Bad rap SMPS have is caused by some manufacturers who use them to lower the cost only(inexpensive implementations).

Yes and no. You have to know what the weight difference means.

If someone shows me a 100Wx2 class A amp but it weights only 30lb. I know it can't be good because he must be cutting corners on heat-sink and power supply, two critical components which weight a lot.

On the other hand, if someone build an amp with extra thick panels which added 10lb to its weight, that extra weight has nothing to do to how it sounds.

Very fair comment. . . that's why I stated earlier that perfectly legitimate but differing technology decisions lead to differing paths toward sonic excellence. . . and differing weights.

I do confess that even I used to give the beefiness of a device major importance in my component choices. . . yet I experiencemore and more that the reassurance that a hefty component can give me can be misleading about its actual sonic capabilities. G.

Open the amps and look in. I always do and find no correlation to what you are inquiring about. Same as 10,000 dollar paint jobs on speakers vs stain. If you know what to look for, subtracting the jewely makes a huge difference to what is really going on. Right down to the capacitor.