I finally received some good teflon-coated wire (OFC with high silver content) and installed new Neutrik XLR female jacks. The XLR posts, as labeled on the jack chassis, are for:
(1) Shield wire;
(2) Non-inverted-signal wire;
(3) Inverted-signal wire.
The wires are soldered to the RCA equivalents, as follows. The shield wire (1 above) and inverted-signal wire (3 above) are jointly connected to the RCA ground (negative or outer chassis). The non-inverted-signal wire (2 above) is connected to the RCA positive (or center).
Once the wire is soldered to the XLR jack posts, there are two ways you can attach the other ends of the wire to the RCA circuits: (1) Use short jumper-wire lengths from the XLR jack andsolder directly onto the female RCA leads along with the preexisting wires on the RCA (leave them intact so the RCA's can still be used if you sell the amp or want to use them later); (2) Use longer wire lengths and solder the XLR wires directly onto the circuit board (leaving the RCA wires intact); if the solder points are not labeled positive (+) and negative (-), you can determine which is which by tracing the wires from the RCA jacks. I prefer this method because it allows the use of upgraded wire and is more direct to the circuit board. If desired, you may 3-way braid the XLR wires before you attach the XLR jack to the amp chassis.
If you use the RCA jacks, you will NOT need to insert a 1-3 jumper wire into the XLR jacks because the input circuitry is not truly balanced. As for the question of whether it is worthwhile to install XLR's this way, it depends on a variety of factors. First, if you do not have preexisting ports for the XLR jacks, you will have to drill large holes for the jacks and small ones for the mounting screws (they do not have to be threaded if you use small nuts to secure them). If you have the ports (like my Aragon amps do), labor aside, you can attach two high-quality XLR's for less than $15 and get a better connection, compared to using XLR-RCA adaptors, which cost about $20 each for good quality. For short, shielded interconnectors, the increase in gain and sound quality with the XLR installation will be small, possibly negligible, considering how much gain in quality is needed to keep it noticable after acclimation. Face it, if you have to do a direct A-B comparison and study the difference in sound quality, there isn't a meaningful difference. If you can easily tell the difference by memory, there is. Anyway, I hope this discussion is useful for those of you that might want to add XLR's to amps with non-balanced inputs. Thanks to those who contributed to this discussion.