Slight increase of tracking force = Big Improvement

I have a Kiseki Purple Heart cartridge on my SME 20/2 with an SME IV mk vi tonearm.  For a while now the sound quality I was getting from my analog rig was less than satisfying.  It didn't sound bad, it just didn't move me the way it had in the past.  This was not always the case.  I can't explain why there was sound degradation.   

I thought my cartridge must be worn out and started shopping new ones (still haven't ruled out an upgrade, but that's another story).  Then, last night I decided to up the tracking force a bit to see if that would help.  I had been running 2.0 grams the entire time I've had it, and that seemed to be great for most of the time I've had it.  I adjusted it to 2.25 grams and immediately noticed a huge improvement.  It was night and day.  Everything is a lot better now.  Imaging, dynamics, bass, midrange and treble are all dramatically improved.  

It's funny because I had never given tracking force adjustments any serious consideration in achieving the best sound from a turntable.  I figured it was best to set it at the lightest recommended setting and as long as it sounded good, don't mess with it anymore.  This revelation is something that has changed my strategy going forward when setting up and tweaking in the future.  

The Purple Heart specs recommend tracking force between 2.0 and 2.6 grams, with optimum force being 2.4 grams.  I am planning to experiment some more in the next few days.  

172b5c3b ea6a 496a 90b0 a5fe6f5abfbbsnackeyp
If you listen to vinyl every day I would suggest revisiting setup once a year. Things change with carts over time and little tweaks to all the settings are important....
Is there a rule of thumb - if the cart is new, it may be used at the lowest specified setting, but after it becomes more worn, it should have a bit more weight? Up the weight a tiny bit. I associate this with the cart sounding "fresh" (but maybe a bit hard) at first, but later needing some adjustment. I cannot get - fully - the fresh airy tone, but it still works well, maybe settling a bit deeper in the groove. With some adjustment. Once a year - or twice.
Kiseki Blue owner here.
I've always maxed the manufacturers settings for carts.

Can't say I can hear a difference between the recommended settings.

Maybe a good thing-I avoid audio nervosa
I have a pal who sets his tracking weight at the highest recommended VTF because he prefers deep bass and warm sound. I find music sounds veiled and muted at high settings, and can't understand why he likes it like that. I track at the lowest end and find it airy, spacious, transparent with cleaner snappier bass. De gustibus non est disputandum!
I always start out at the middle of the recommended range and dial in with small increments going both ways and no I don't always hear the difference between one small adjustment but always do with at least 2 in the same direction letting me know that I am headed the right way. Sometimes I even exceed the max suggested range but only by 10 to 12 %, this has been the case with my DRT XV1s and my Koetsu RSP but most the time find the ideal spot at max or just below. This is one of the things I love about vinyl, when you get it dialed in, WOW.
As a side note, the SME tonearm is really great at locking in the VTF setting.  Just flip the lock lever open, turn the small dial the direction you desire to reach your target VTF, and flip the lever to lock.  I set mine at 2.0 grams a year ago and when I checked it yesterday it was still at exactly 2.0.  Tonearms that have a simple threaded weight and no lock feature will come out of adjustment relatively quickly.  You need to check VTF monthly at least with those.  
@noromance , you know what you said about the difference between the lighter and heavier settings is spot on and you can tell from my response that I fall on the darker side of things, with that said I use two different record clamps on my system, the BDR 1 piece clamp and the Stillpoints LPS clamp/weight, and they do the exact same thing, the Stillpoint is the one that has the flavor you describe. Just thought that might be worth mentioning.
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Dear @snackeyp  : There are at least 3 main premises why to follow the VTF manufacturer/designer LOMC cartridge advise:

first is that following his advise cartridge coils will stays " perfectly " centered and second is that tracking cartridge abilities are spot on ( cartridge suspension is working as designed to. ) as the designer desing it for tracking.
Third and maybe no less important is that the designer cartridge voicing did it at that specific VTF.

Regards and enjoy the MUSIC NOT DISTORTIONS,
A slight hardening of the suspension would require a slight increase in VTF, so that "cartridge coils will stay 'perfectly' centered", as Raul put it.

You say "It didn't sound bad, it just didn't move me the way it had in the past." This seems consistent with your findings: the Kiseki is an older cartridge, and your slight .25g increase restored the original positioning of the coils. 

I own a Lyra Delos and even though the tracking force they recommend is 1.75 with a range of 1.70 to 1.80, I track at right around 1.70 and feel it sounds best at this setting.  Other cartridges may vary in this area.
@tooblue I find that clamps and damped mats, soft feet etc. rob the life from the sound. It could be argued that I am hearing tiny resonances which augment the perception of information. It could also be argued that you are hearing less detail due to the absorption of energy by damping materials.