I just purchased a Laney Pro Tube 100 Amplifier,
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2515833782

and I was wodnering what the best 4x12 speaker cab and speakers to get
for it would be. I play a lot of metal (Iron Maiden, Sabbath, Blue
Oyster Cult, Nightwish, heavy and shredding). I also need to take into
account cost. I don't want to spend more than $300 for a cab, if
that's possible. I don't care about its age, or if it looks like it's
beat to hell, as long as it sounds good.

Hi, I’m going to take a stab at your question. Deciding what is “best”
is always subjective, so you’ll have to follow your instincts there.
But I’ve gone with conventional wisdom out there.

The Laney is a great amp and since it’s a classic tube job I’m
guessing you might be leaning toward a more classic cab with speakers
as well (and you mention going for a used item). So why not go for a
Marshall? Very well made! There are 4x12 Marshalls on ebay all the
time. Granted, they just push your spending limit, but it’s a long
term investment, and they’ll hold their value. If you keep looking I
think you’ll find something just barely affordable.

Your amp is rated at about 100 watts and so double check the handling
of any cab outfit you go for, but usually the Marshall ones that I
know easily handle anything you can give it. Most (or all?) seem to
have Celestion speakers in them. The most revered is probably the 1960
series, which has been made for years and years. But I have no doubt
other 4x12 Marshall versions will be really suitable, too.

Here’s one that was current as of this posting:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2516095907&category=1288

And another:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2515505469&category=38075

Don’t just take my word for it about the Marshalls. Some user opinions
are viewable at epinions, and this link should take you right to a
list of Marshall 4x12 options (note the higher new prices than the
used ebay ones above):

http://www.epinions.com/search/?submitted_form=searchbar&search_string=4x12+marshall&tax_name=&search_vertical=all

Another possible option which uses Celestion drivers as well is the
Avatar. The company has listings on ebay that are almost half the
price of the Marshalls, and you might get similar sound overall.
Here’s one:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2517159739&category=2384

A Crate cab with 4 Celestions is also on ebay, and a bit cheaper:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2516837203&category=38073

Obviously you can’t go wrong with a classic Fender cab, and though
this one is going to go too expensive (it’s a beauty), other Fenders
have ended on ebay under $300.

Melody Exchange is a company with many used offerings that seem to be
linked through ebay, but might offer advice by email:

http://www.melodyexchange.com/me/mexsearch.cfm

If you don’t like the ebay idea, which is usually the cheapest option,
you can try any number of new/used guitar equipment shops on line (or
check locally where you live to avoid $50 shipping). Find a Marshall!
 
A Marshall 1960 “copy” of sorts, which might be much harder to find
but in case you run across one, is the Park G412, and a comparison
site is found here. The fact that the Marshall was copied is the
ultimate testimony to the Marshall:

http://www.blamepro.com/br01400.htm 

Vintage Guitar classifieds, a hit-or-miss proposition worth some
browsing if you have extra time:

http://www.vguitar.com/classifieds/details.asp?userID=Acker&looktype=member

A nice site for used stuff is Chris Guitars:

http://www.chrisguitars.com/amp.html

Another on line guitar firm with a page on Marshalls:

http://www.wholenote.com/resources/revmain.asp?I=1620

Well, I tried to solve your answer by both giving what I would do,
based on some reading and a little experience out there. And I’ve
listed a bunch of sites and options to do your own research if you
want to go further. Please let me know if you have a specific concern
I’ve missed somehow. By the way, matching the amp to a particular cab
doesn’t seem to be an issue (though I’m sure if you talk to enough
people, you’ll find lots of favorite combinations they will swear
by!). Double check your impedances match and you’re set.

My search strategy:

on ebay search (www.ebay.com) I typed: 4x12 marshall
I then clicked on the guitar category to the left and just entered:
4x12
on google: 4x12 guitar used
and: 4x12 marshall
on epinion: 4x12 marshall

-llmnop-ga

Well-reasearched and nicely explained. I am most pleased-- I will
certainly be keeping an eye out for the various models mentioned.
Thanks a lot for saving me frustration, time, and money!