Upgrading the speakers is extremely easy and costs approximately $30. These machines have decent sound in STEREO, something not even modern Stern machines accomplish, so give these machines the speakers that they deserve and get rid of those lousy 15 year old paper cones.

BUY:
Front speakers: Buy 3.5 inch car speakers. Make sure that the mounting depth is 1.75" or less. Make sure the impedance is 4 ohms or greater - most are but now and then you can find a crazy 2 ohm speaker. Don't go crazy with super high end speakers - the audio is good for pinball but not that good compared to, for example, car audio. I purchased these although any $15 pair will probably sound equally good: www.amazon.com/PYLE-PLG3-2-3-5-Inch-Two-Way-Speakers/dp/B0007L8BT4/ref=sr_1_5
Also consider cosmetics - if you want the speakers to be invisible behind the grill, make sure to buy an all black speaker such as this: http://www.amazon.com/PYLE-PLX32-3-5-Inch-Two-Way-Speakers/dp/B0002BEWZ6/

Cabinet speaker: Buy a single 6.5 inch woofer rated at 4 ohms. Lower is likely to cause problems for the amplifier, while higher impedance may actually result in lower bass volume relative to the front speakers. I have tried a few, including a few that were quite a bit more expensive, but keep coming back to this one, which is the closest to being a straight drop-in for Williams machines: http://www.amazon.com/Pyramid-WX65X-6-5-Inch-Injected-Woofer/dp/B000EFMPCW/ref=sr_1_4

PROCEDURE
1) The hardest part is just removing the front speaker grill and it's not hard at all. There are 3 nylon nuts behind the wooden panel. These can be reached with a 11/32" crescent wrench once the translite is off. In the picture below, I placed a crescent wrench on one of the nuts.
P2281210-688.JPG

There are three T15 security torx screws on the bottom of the speaker grill. If you don't have a security torx set, here's a cheap set: http://www.amazon.com/SE-Bit-Security-33pc-7519SD/dp/B0009U6AB4/ref=sr_1_1

2) Once the grill is off, installation of the 3.5 speakers is straightfoward with the removal of two screws per speaker (re-use these screws to install the new speakers):
P2281211.JPG

The wiring connectors are fool proof. Simply pull off the female spade connectors and plug them into the new speakers. The connectors follow car audio conventions whereby the negative terminal is narrower than the positive terminal.

P2281212.JPG

More pictures are available from NewPinOwner in this post: https://pinside.com/pinball/forum/topic/rfm-hillary-come-here-you-gotta-see-this/page/5#post-1828630

3) The cabinet speaker is clearly visible and easily removed with four nuts. I prefer to cut off the soldered wires and crimp on female spade insulated connectors for 14-16 gauge wire. These spade connector simply slip on to the terminals of typical car stereo speakers. For those interested, NewPinOwner has more details on installing the 6.5" speaker: https://pinside.com/pinball/forum/topic/rfm-hillary-come-here-you-gotta-see-this/page/4#post-1814614

There is not necessarily a correct polarity - just hook one wire to each terminal. If you want to spend the time, listen to the audio (with the whole machine put back together) with the wires one way and with the wires swapped. Whichever seems to have slightly greater bass is the "correct" polarity although the difference will be extremely minor at best for most ears.

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That's it. The bass is significantly more noticeable and the upper midrange is much clearer.

The initial post recommended Pyle PLG3.2 speakers which have a yellow frame. For those who do not want the speakers to stand out visually, NewPinOwner has reported that the Pyle PLX32 work equally well and are black: http://www.amazon.com/PYLE-PLX32-3-5-Inch-Two-Way-Speakers/dp/B0002BEWZ6/

My experience is that most any 3.5" speaker will sound great (just be sure to check for a sufficiently shallow mounting depth). It is the 6.5" cabinet speaker that can be very hard to match and for that reason, I continue to recommend the Pyramid WX65X for the 6.5" cabinet speaker.

If I lived so close to Parts Express, I would be much poorer.

I think one should normally purchase high efficiency speakers. However, if you like bass, you should purchase low efficiency speakers for the head and high efficiency speaker for the cabinet, which will have the effect of making the bass driver relatively louder. (BOOM, BOOM, "16th President Wins")

I haven't had much luck with either Boss or Pyramid as main speakers, but have had great luck with Pyramid 6.5" bass and mid bass drivers for the cabinet.

Thanks. Updated the first post with this info.