Different Tubes and general purpose?

Thanks millercarbon.  I ask as I’ve read plenty of articles about how there was notable, not step change, improvement in overall sound quality.   I get that that a lot audiophile purchases can either be snake oil or nearly inaudible changes.  
I have spent, at least in my world, a decent amount of $ on my system.  I’m largely pleased but I seam to be chasing the perfect balance and execution of several instruments in certain songs that seem to be not as crisp or prominent as i know they were recorded.  I would like to explore “tweaks” (i guess not upgrades) to my equipment that I’m hope can help me achieve what i want, without wholesale swapping out of equipment.  
My Harbeth HL5 plus sound incredible (coupled with a sub) on several albums (Fleetwood Mac, Tom Petty, New Order - Movement, the smiths - Hatful of Hollow), but on others (Big Country - the Crossing, or Led Zeppelin - 1st album) they seem to fail in the a really crisp and distinct mid range.  I could be the recording, but then again it may just be that these are speakers better suited for classical or jazz.  I was trying to explore other system changes to tease out more. 

*best enhance.  

Thanks all for the practical advice.   
I’ve had my amp for 2 years now and it probably has 50-60 hours of use, so I would have thought it is been broken in.  I will call my distributor and ask what he thinks in terms of where to start on rolling any tubes.  I’m not in a rush, so I have time to play around. 

Facten thanks for the constructive help.  I have had a nice power conditioner for a few years now and have noticed a small improvement...really just eliminating any very faint background noise that might otherwise degrade or mute certain frequencies.   I have not invested in a new power cable, but will explore that at some point in time. 

Your advice on tubes is hugely helpful and I’m very grateful.  

Thanks plga. I will take your advice.  Much appreciated.  
Also thanks imhififan.  Is there a decoder ring for rectifier?   I think i get the others, but still not sure what role each plays in sound production.  

Thanks Dekay for the follow up.  That has yet to be answered and i honestly tries to research this, but struggled to find anything authoritative.  It would be helpful to know. 

Wsrrsw 
I’ve perhaps underestimated my hours, but not by much.  On average i may get 6-8 hours a month, but with the current quarantine its maybe 4-5 hours a week.  
Room is large with 14ft ceiling.  Speakers are Harbeth HL5 plus.  Power seems fine and I do use a conditioner, contacts all fine to the. Eat of my knowledge.  All equipment is 3-4 years old.