insulating after-market speakers

First off, I am a novice - thanks for your patience. I've had some luck "tightening up" some boomy subwoofers by lining the cabinet interiors w/high-grade speaker insulation(egg-crate style) purchased from PEx. Of course, these were budget subs. I am presently awaiting the UPS arrival of a replacement tweeter for a budget bookshelf speaker(ported in both the front & rear) and I'm eyeballing the naked interior of this cabinet. Very little insulation was provided. I realize that cabinets are tuned a certain way upon design but I ask the question:"Can I improve the sonic quality of the cabinet by lining it's interior w/a high-grade foam insulation as I did my old subs, or am I more likely to alter the sound in a negative way by changing the interior cabinet dynamics therefore altering the cabinet's tuning?" Of course, I would do it in a manner that would allow air flow to both ports. Somebody (w/a great deal of knowledge) suggested using dampening spray (usually used for car doors) as an alternative to foam, but I think he made THAT suggestion NOT KNOWING I can easily access the whole cabinet interior at this point in time. What are your thoughts on this? Foam insulation, dampening spray, or even the third alternative of using vinyl self-adhesive dampening sheets. Thanks for your input!
A long time ago I bought a pair of Wharfdale Diamonds. They were highly regarded, but I live in a relatively small town so auditioning was not an option. I sold the speakers I had to fund the new purchase.

When the speakers arrived I tried to be hopeful though they were smaller than I expected. I hooked them up and let them play for a couple of days. After enough time to break in I sat down to listen.

I almost cried, they sounded awful! I didn't know what I was going to do since I did not have money to buy something else, and this was long before AudiogoN. I decided a fix was more rational than suicide.

I opened the cabinet and rewired the whole thing with some AudioQuest cable I had left over from something.

I gave them a second chance, after appropriate break in. The speakers sounded a lot better. Good enough that when I had other 'audiophiles' listen to them blindfolded they were surprised to see how small they were.

To get back to your question; your speakers can sound much better with a little effort put into the cabinet. Polyfil is a good start, bracing can help, and using some good cable inside is worth the minimal cost. Try any or all of them, you should be pleasantly surprised.
Thanks Uppermidfi for the helpful anecdote. Now, any others on whether better insulating the cabinets could improve/harm the sonic qualities of the speaker in question?
One good thing about doing it yourself is that it is just as easy to un-do. Give it a try, I would!
What frequency does the woofer crossover to the mid ( 3 way ) or tweeter ( 2 way )?

Different materials will affect both bass characteristics AND midrange response. As such, damping a speaker properly for best results requires using the right type, amount and placement of the materials used.

If in doubt, here's a "generic" approach that typically works pretty well with most vented designs.

Line the walls with foam. Lining the walls will help reduce "cabinet chatter".

Fill the upper cavity with foam. Filling the upper cavity will:

1) improve bass definition by slightly reducing the amplitude of peaks at resonance

2) smooth out the response in the upper bass / lower mids where the ports typically contribute output that is undesirable

3) extend bass response by a very few Hz.

Using sheets of foam offers the advantage of being able to cut to an exact fit and remove / add as needed to achieve the tuning that you desire. Sean
Just reworked the acoustics in a church recital hall. Wound up using long hair sheeps wool.This material is easy to work with. Easy to weigh and to pull apart sections in a linear and repeatable manner.. Acoustically it is very linear in how it reacts to all apparent frequencys. No suckouts, it doesn't seem to grab and hold any certain band in an unpredictable way. Replaced the dampening material in a pair of Harmonic Precision prototype speakers again with the wool. Improvements in mid-bass and lower vocal range were the result..More linear and coherent.Tom
Tom's suggested use of long hair sheep's wool is probably the best material that you can work with. As he stated, it is very linear in absorption across the frequency range compared to most other materials. Next comes fiberglass, but it has to be quite thick in order to achieve good absorption at lower frequencies.

It is due to this non-linearity that i brought up the crossover point on the woofer, etc... Some materials will work better than others over specific frequency ranges and / or one can tailor specific non-linear absorption characteristics to their benefit. Doing so can result in a system that is truly "tweaked out". Sean