low volume

I have a ST120 that's about 4 years old and a just built the SP12. I seem to have a lot lower volume with it verses the SS preamp I was using.
Not sure why as everything is quiet and it runs fine and all voltages check out. Tried another set of tubes and no change I still max out the gains.
But with the tubes 4 years old on the amp could it be time to change them out ? I was going to get another set anyway and I was just wondering.
They have had a lot a play over the years. The rectifier tube pops from time to time and I just keep a spare. Would this be a normal life for the
KT88 ?
You thoughts and thanks

If you average 2 hours a day, your KT-88s have close to 3000 hours on them. I would definitely advise a new quad. 2000 - 3000 hours is average useful life. Rectifiers popping is often a symptom of power tube failure!
I've made a pledge to purchase a new quad of Genalex KT-88 EVERY YEAR. That's about $250 which I can easily budget for, and buy well in advance.
One of the difficulties of old power tubes is that often the degradation is quite gradual and our ears adjust to the slow decay of sound. Sounds to me like you are fast approaching the day when there may be a catastrophic failure rather than a slow one. Caveat Tubester!

I would say if there was a noticeable change in volume of your system in merely changing between preamps -- an A/B comparison so to speak -- that would not indicate a problem with the power amp. It appears there is a difference in the output of the preamps and would start there. Saying that I have no idea how to determine the differences without some test equipment. Seems you need some way to measure the line output of each preamp in volts or milivolts or microvolts or whatever and compare them I think. And I am not sure what "range" a typical power amp wants to see in terms of an input signal. After you solve that it could be that new tubes are in order. I always remind you I am not a tech guy but would hate you invest in new tubes only to have the same problem.

Before doing anything, what is the rated output of the unnamed mystery solid state preamp?  And how does it compare to that of the SP12?  Some manufacturers build excessive line level  output levels into their equipment which impresses the "Gee-Whiz" crowd, with the infamous, "I only have to turn it up a little to blast you out of the room, imagine what POWER it has all the way up!"

Answer: exactly the same as the more conservative design as long as both can drive the output stage to clipping.  Once the output stage is clipping, turning it up more may seem louder, but it's all distortion.

Check out the specifications.  

Also the taper on the volume control in a preamp may be abrupt in the first part of rotation to impress the same "Gee-Whiz" crowd.

My question is....does it get loud ENOUGH or does something just seem wrong? Does one unit start to clip or crap out before the other when you get to higher volumes? If it seems it gets loud enough for listening and even when you want to go a little nuts, the unit is OK?? But if something seems awry and if it is too low for normal or occasional crazy listening it's an issue with the output level and you may need to dig deeper, at least compare specs as is suggested.

Another guitar amp story, but relates to this situation..the Fender Hot Rod DeVille is a newer design, 60 watt tube amp that gets to max volume between 3 and 4 on the volume scale. Story is they wanted kids in music stores to bang on it, see how loud it got on only 4, and think, holy crap, think what it will do on TEN when i get it home!!! Unfortunately it had little gain after that and pretty much topped out. It was a good amp but a bit of a fraud, hard to control for serious players and didn't have the range on the control pots you really need. It was all in the way they had the preamp stage set up. I am reminded of the Spinal Tap Marshalls that went to "11."

I did a test . I used a lap top as a pre , through the dac to the amp. Using ITunes on the lap top , but tried the cd player and is no better. The ST120 has the attenuator installed so works fine this way. The long and short is the ST120 has more than enough jam hooked up like this so the problem is the pre.
I did get the upgraded attenuator and caps. Trying the different caps makes no difference. If you max out the attenuator to the last peg you get a blast so seems like more power to be had ? And maybe I am asking too much just most equipment I have ran usually tops out at 1:00 on the attenuator and you can go past but you will start to distort. This amp never seems to get there ???
Yes there is enough volume per say but I am maxed out on gain and volume to be at a level I like when rocking it. Hooked up as mentioned above the attenuator is only about 3/4 to distortion on the ST120 and you would be leaving the room. That's why I find it strange. I don't know any other owners out there with the ST120 / SP14 combo to hear what they have to say.

Despite my rant earlier about your probable need for new KT-88s in the ST-120 (which I still believe), it now appears from this new evidence that there is a definite problem with your SP-14.
I use this equipment too. And I have the same attenuator and cap upgrades. I generally run the preamp high - at around 3 o'clock - where its tone-enriching capabilities are at maximum, which means usual position of the ST-120's attenuator is noon to one o'clock - two o'clock for real loud, earth-shattering sound. so there seems to be good cause for believing that something is amiss in your SP-14.
If all else fails, talk to audiobill. He can fix anything.

Thanks I will try Bill

are you saying that you can't turn up the volume enough so that it's loud enough to blow you out of the room?
Do you have both gain controls and volume control turned up all the way?
With your preamp here, normal listening level was about half volume, and it was super loud at full volume.
If you'd like another opinion, let Bill Gratrix have a listen, and between us and more info from you about your source (I use a CD as a standard source)
we should be able to get it right for you.

Did this issue ever get resolved? I was curious as to what may have been going on here. Any updates? Thanks...IA