How much does power amplifier really matter comparing to other hi-fi components?

Source is most important. The sound can only degrade from there as you go through the signal chain. So turntable (arm/cartridge/phono stage/table), integrated amp (keep as many interconnects out of the chain as possible), speakers. Of course you want to get speakers you like and ensure the amp has the power to get the most out of them, but this is not the typical advice you get from dealers as the margins are higher typically with speakers than the other parts.

And don't forget cables, isolation treatment if necessary, and room placement.

It is impossible to put precise percentages in each area, but be logical. If you have a $10K budget, don't spend $8K on a turntable unless you are planning on upgrading the other stuff down the line. Have an end goal in mind, and then if you are like a lot of Goners, the end goal never stays the same. 

cclee2022     If you are looking for a great pairing for the XT2 I would highly recommend the Lumin X1 and  AMP. I had this combo with the XT1, TD1.2 and TD3.2's. Outstanding synergy..

I think the biggest flaw in assigning a budget % to each piece of the audio chain is that it doesn't take future changes/upgrades into account.  Here's how I would approach building a systems based on my limited experience.

First, try to figure out what a realistic goal is for your system.  When I purchased my Focal 836v speakers I had the opportunity to listen to nearly everything in the Focal lineup from the $500 bookshelf speakers to the Grande Utopia EM speakers.  I never considered that a $30,000 speaker, for example was a realistic option for me and ended up selecting the 836v that was the most expensive speaker that I could afford.  I have always liked the Focal sound and have never regretted my purchase.

My speakers were initially paired with the SACD player (still use) and Onkyo receiver that I already had.  I almost immediately upgraded the receiver with an Integra receiver (the goal was to still be able to do home theater) and I still have this receiver in my home theater setup.

I eventually added a stereo amplifier and have had several different ones.  My current Pathos Classic One MkIII (with vintage Mullard tubes) seems to be my "forever" amplifier.

I recently did an op amp upgrade on my DAC which gave me some perspective on how significant the role of a quality source is.  I have a little bit of pro audio experience and know that gain structure is important and in a practical sense means that any flaws in the signal just get more amplifier at each step in the system.  If your source produces a noisy signal then your preamplifier and amplifier can only amplify the signal they are given and a quality speaker will expose it.

The issue with starting at the source is that the speakers plays the most significant role in the final sound of your system, so if you don't like the characteristic sound of your speakers I don't think a perfect source will change your mind.

I would recommend investing in a speaker that you love and is expected to be capable of achieving the sonic quality that you desire.  I had the opportunity to listen to my chosen speakers in the main room that was setup for the Grande Utopia EM speakers, so was able to evaluate what they were capable of.

I was also able to listen to the $500 bookshelf speakers on the same system and even though they sounded pretty amazing, I could tell that I would want more than they could provide.  

It's a journey.

So now your saying I should make it now 20% for my cables....good bye.

@limomangus - If you're not willing to give 120% or more then you're not trying :-)