Cheap tweaks...What would YOU reccomend?

For the tightest bass, drive self drilling screws directly into the sub-floor (through the carpet), and rest your speaker spikes on the screw heads. You will need to pick screws with a wide nut type head, otherwise it is "very" difficult to line up the spikes with the screws (I did it once on philips heads, but it took some work, now I have a hex type head screw which makes it a little easier). When you move speakers that have been sitting on carpeting, you will see the marks left by the spikes (on most carpets anyway...)which makes it easy to position the screws.
Level the screws (rest a flat ruller on the screw heads between front screws and level, repeat for rear screws, check rear to front). Place speaker and adjust spikes for correct level (I like mine slightly tipped up in the front).
It seems like a lot of work and you must not care for the sub-floor under the carpet but man, you will end up with the tightest, most tunefull bass your speakers are capable of. This also helps with imaging as the speaker is better anchored. It is a big improvement in my experience. Just make sure you have settled on the final location for your speakers before you do this...
"Disclamer: not liable for any damage to floor or fights with significant other as a result of this tweak!"
Cheers.

Take some time away from tweaking your system! This the ultimate tweak because it calls on you to deal with your music directly-1:1 time. Questioning whether this Music sound better having done this and that- and then that and more, really seems to pale compared to questioning whether you really like this music you have worked so hard to improve. Is this music worthy of a boombox,discman, or bad accustics, or whatever desperate lengths you just might go to just to be able to hear it one more time? Also, time away from such nagging analysis allows your ears time to hear your system in the raw & therefore the possibility of that enhanced clarity that will truly allow you to assess the strenths and weaknesses of your system and the various values of your musical preferences. So, my suggested tweak- give yourself a break!

Nice Thread!

The "cheaper" the unit, the better the tweak "sounds". (Mine are cheap)
These are some of the tweaks that I recommend so far to my friends:
vibration damping: place bike's pneumatic bladders(I think this is the name), softly inflated, under the component, and books or other similar on the top of them. It looks horrible but: no "congestion", better dynamics, more integration.

Put ERS sheets (cryotweaks.com) over the power section of your components. Internally works better. Takes away lots of noise. It can take away dynamics if used on amp section.

Instal IEC receptor and try some power cords if your component has "captive" PC. Very effective.

I'm waiting for Bybee's purifiers to arrive. That is supposed to be the supreme tweak!

Tweaks can cool down your fever to upgrade.

The supreme tweak: Bybees purifiers!
I put one on each positive speake cable lead and totally
transformed de sound! read the reviews. They are absolutelly wright! It sounds like if the speakers became more sensitive and represented a smaller load to the amp. But more: integration of transducers as if the crossovers were upgraded; easyness to the sound; attacks are fast and decays are "real"; top to bottom coherence; detail, detail; it's easy to follow the musical lines...etc.

If you want to upgrade speakers you'll have to spend twice as much as you paid for yours to hear an improuvement. It's much cheaper to install Bybees! And you still can keep them when you finally upgrade.

I don't know if they can be called "tweak" o "cheap". They are expensive in absolute terms but when you hear the effect they produce you will soon forget the price paid (but if you know of a sale, call me). And they are said to be additive(and addictive) in effect.