Tube Rolling for Pre-amps?


I have an Audio Research LS-16 that feeds a Madrigal Proceed HPA2 solid state amp. 

I did not choose this equipment; it was given to me. From what I can tell the original owner spent a good bit of time matching up these two components with the speakers, Aerial Acoustics 7Bs.

The tubes in the AR are original. Probably 10-15 years old? But with an extended period of not being used.

The system sounds good to me. I don't need to change anything.

However, I've been told that the tubes are old and may need to be replaced soon. I've also read a little bit about tube rolling but most of the articles I've seen talk mostly about tube rolling with amps and not pre-amps.

So I have some questions:

1) If the original owner wanted a system that sounded like a 'tube' system, why not have a tube amp and pre-amp? Does it make sense to have a tube pre but SS amp?

2) Other than outright failure, how will I know if my tubes _need_ to be replaced?

3) With a SS amp, will changing tubes in the pre-amp have a noticeable impact on SQ? In other words, is tube rolling for a pre-amp played through a SS amp a thing?

4) For someone like myself who is happy with the system and who is also easily frustrated by A/B testing is tube rolling the pre-amp even worthwhile? I can almost always hear a change when comparing various components but only rarely am able to say A is better or worse than B.

( I probably can't even articulate effectively what I look for in terms of SQ except that I like the bass to be tight, precise, well defined and not boomy. I do not like the music to be overly bright or tinny. I like depth and lushness but not overly warm. Not sure if these are things you alter with tubes.)

Thanks for any advice.


n80
I'd like to mention that a 7dj8 isa great choice for you. Someone else also said this in passing. It's important to study the equivalent tube types, especially if you're on a budget. Preamp tubes can have a profound impact on the overall sound of a system. 
@n80  The National 7DJ8 platinum is also what I just got for my BAT (replacing Phillips 7DJ8s which were no longer available).  I'd say that's a pretty solid recommendation.  Dick
Think the tube pre and SS power
covered above (also what
i use) pre really defines the sound.
and tube rolling in pre doe make
a differance most of all in a phono input stage as levels lower. I switched fine JJ ecc83
for telefunken and then a Siemens 5751 tube and the midrange opened up even more. And then the sound was good and I stopped.
 
If you like the system then just enjoy it
@n80- You got me thinking about my tube inventory. I found a NOS pair of these(the exact same vintage/iteration tubes as in the description, except- the printing on mine is still perfect): https://www.ebay.com/itm/E88CC-6922-AMPEREX-N-Y-USA-GOLD-PIN-TUBES-JAN-USN-MATCHED-PAIR-1960s-MDGGZ/... If you like, I’ll send them(gratus) and you can experiment. They were a tad too warm(for my tastes), when I tried them(maybe 17 years ago). Even if you don’t care for the sound, it should let you know how much impact tube rolling can have.