Do I need speakers upgrade?

The Proacs are easy to drive so your mc152 should be able to drive the fine. Hopefully you can try then first.

audioman58, this may sound native, but would it help if the stock components - particularly the caps, are replaced with higher quality but equal spec units, or modding requires in depth knowledge of the circuit and modifying the stock design/specs?

You have not really let anyone know what improvement in sound you are looking to achieve.  Larger Proacs will give you a bigger Proac sound.  If that is what you are looking for, then go that direction.  What about soundstage, what about total dimension, what about the TONE, air around vocals and instruments, instrument placement, definition, decay of notes?  What does piano sound like, what does the sax sound like, what does the violin sound like?  IMO your front end is the weakest link.  How did I determine that, well I build DACs, preamps and phono stages.  These custom made products do certain things that most people have never heard before.  They bring out the systems potential.  I don't know what you expect your system to do, but without all that information, it is hard to make a specific recommendation.  I can change the sound of your system with just a few resistor changes, cap changes, or adding a choke.  It could be that simple.  Happy Listening. 

Hi bigkidz

When you say my front end is the weakest link you mean that my current speakers are the weakest link in my set up ?

My room is larger than yours (see my system page for full details on room and components) and I think I have plenty of speaker for where I listen.  I only listed to the ProAc Studio 125 (beautiful midrange) and ProAc D20 before I ordered my 148s.  Why did I go with the 148s?  They have the same (non-ribbon) tweeter as the current D response line, same great midrange (125 still had better) and have better low end control with the 2.5 way design.  I've heard the 148s are a more refined version of the 140 mkII as they went to different drivers.

I would consider the following upgrades for your 140 mkII ProAcs:

1.  Get some Soundocity SEV9 outriggers.  I use these and are much more rigid and stable than the factory ProAc spikes.  The SEV9s are also flexible enough to use with other tower designs.
2.  Upgrade your speaker drivers and crossovers to the 148's configuration.

Does anyone have recommendation for crossover upgrades for my Studio 148s?  I would love to get my 148s even closer to the 125's midrange with an even more refined sound.