I need to ask what power tubes are you using in the RA100 combo? This I found to be the key with the Royal head or combo as I have both. I have tried many different power tubes in this amp ranging from the stock tube STR447 (vary harsh sounding), Mullard Reissue EL34 (agressive) Tung Sol EL34B (b=bland in this amp) and a few others not worthy of noting. However, most impressive tube I have ever used and for the past 6 years in the head were the SED =C= EL34. Those put a smile on my face. I was disappointed to hear of the SED closing down production on consumer grate audio tubes. You can still find them like at places such as TheTubeStore. However there is a much better resource for these at reasonable price (still expensive but not a raping cost) is Mesa. STR442 SED =C= EL34 are available from their on-line store. Mesa branded and also has the SED =C= branding as well. I have bought a few of these and have a quad in my Combo.
Now for tuning the preamp section. You can basically leave it all stock tubes or opt to tighten up the Red channel and improve the blue with two preamp tubes of merit. They are of the NOS category but worth the expense as they fall in the low price point compared to the highly sought after tubes of that era. V1: NOS RFT/Siemens 12AX7/ECC83 and for V2: pre war Ei CV492 (often sold as a counterfeit Mullard long plate) Still that Ei tube sounds great. Why V2, it is the second gain stage for the blue and the tone stack for both red and blue. The RFT is similar to the JJ ECC83s in some respects but when the gain goes up and that added gain stage on the red channel the character of the beast is revealed. Bass remains tight and tone is well balanced. I am almost getting a Recto tone without the extended sub harmonics of a cold clipper circuit you get with the Recto. With the =C= EL34 and the two NOS preamp tubes this amp will give you that vintage vibe as well as some modern tone that cannot be found with other amps, including the TC series. After doing all of this experimentation, I had gone back to the original V30 speakers in my RA combo. I also use the combo with a Recto horizontal 212 cab to make a 412 combination but 4 ohm total load. I almost like that more than the RA100 half stack. Now with the multi-soak in use you actually can hear the tube saturation even on the clean channel. I never noticed it with the stock STR447 tubes. As for the Redbacks, I only have the one 8 ohm to try in the Mark V 90W combo I was having brittle ice pick tones on the lead channel that I could not tolerate. Redback did cut most of the upper frequency out of the mix as will the Celestion Cream 90W Alnico speaker. I personally did not admire the characteristic sound of the larger voice coil. Mind you that speaker will take abuse quite well as I tried to kill it with the JP-2C since it peaks at 140Wrms. That did help to break it in some, still it lacked the upper frequency range and took away character I would find appealing. It was as much as a blanket as the Creme 90W but did sound much better on the clean channel of the Mark V. Did not like it with the Royal. Sure I will hit a new speaker with everything I got to see if it has potential. I can see the point with the V30 in an open back cab as there may be some fizzy character at reduced volume. I could tell that the speakers that were in the combo were replaced due to the crappy soldering on the terminals. Looked like some cheap solder from a hardware store and had grainy appearance and too much which is an indication of poor soldering skills. I had cleaned off the cheese and soldered the terminals properly with a 60-40 tin lead solder and that made a huge difference since the solder joint was done properly. If you are after a more bluesy characteristic, the Creamback G12H75 were a good choice. I did like that in the Royal Combo but the tradeoff a lower volume. A blend of the Creamback and Cream 90W alnico (both 16 ohms) brought back some of the lost volume with a hint of alnico chime I could not get with the pair of Cream 90W speakers. It was not after restoring the amp with the V30 speakers is when I question why I took them out in the first place. I would personally start with the preamp and power tubes first before removing speakers. If you like the Redback tone it is up to you. I did not look at the link you provided yet but will after I submit the post. Keep in mind there are tonal differences between 8 ohm and 16 ohm speakers of the same type. I have also heard some great sounds of blending the CB H75 and CB M65.
