Agree with above. Small speakers played in a large room will always sound wimpy - no matter what volume they're played at. For an accurate tonal balance with decent impact, the speaker needs to be a good size match for the listening space, roughly speaking. But if we're talking only about maximum volume...
The three main parameters that affect maximum volume available from a system are:
amplifier power output
speaker sensitivity/efficiency
speaker power handling
As far as maximum system volume goes, the higher these figures the better.
one is 2 x 30 watts @ 6 ohms and the other 2 x 60 Watts @ 4 ohms what differences should I expect.
Quick answer: IMO, not a lot - but an individuals interpretation of volume is subjective. Check this out.
Discover the smallest difference in level (dB) you can reliably hear.
www.audiocheck.net
These are generalisations.
Amplifier power output:
Most transistor amps manage to output more power into lower impedance speakers - but only up to a point. When a speakers impedance/resistance drops to a really low value, many amps struggle and, typically, either start to loose power or overheat or blow a fuse. Most amps don't like it when a speaker's impedance is below about 3 Ohms (although some amps are OK - depends on the amp design).
If the two amps you mention specified power output figures (Watts) into the same speaker impedance (Ohms) then you could compare them directly and determine which amp would output most power - but you can't because the speaker impedances stated are dissimilar (i.e. one is into 6 Ohms and the other into 4 Ohms). Best guess is that the 2 x 60 Watt into 4 Ohm amp would output about 2 x 45 -50 Watts into a 6 Ohm load. This is a guesstimate based on past experience and depends on certain amp design factors of which I'm unaware.
So we have two amps. Into a 6 Ohm load, one amp outputs 2 x 30 W and the other probably 45 -50 W. That's equivalent to a volume increase of about 2 dB. Doubling amp power into the same speaker gives a volume increase of about 3 dB which would give a noticeable, though far from large, volume increase - all other things being equal. Note: there are exceptions, which again depends on amp design.
Speaker sensitivity:
The Monitor Audio BX1 speaker allegedly has a sensitivity of 88dB @ 1W @ 1metre. I say 'allegedly' because speaker manufacturers often overstate specifications of their speakers, particularly it seems wrt sensitivity. But let's take the Monitor Audio figures at face value. The BX1 is an 8 Ohm impedance speaker, allegedly, and has a power handling of 70 W RMS max. Perceived volume is measured in decibels (dB). Using these figures, we can calculate the maximum volume this amp + speaker combination should achieve. The speakers give 88 dB sound output for 1 W input. Every doubling of power input increases the speaker's volume by 3 dB (approx.) which translates to 91 dB for 2W, 94 dB for 4W, 97 dB for 8W and so on. If the amp you currently have outputs 2 x 30W into 6 Ohms, I estimate that would be approx. 2 x 25W into 8 Ohms (i.e. the MA BX1 impedance). An amp of 25W equates to approx. 102 dB of sound output from the BX1. On it's own, this figure isn't particularly useful but it can be used for comparisons. Take the next Monitor Audio speaker in the BX range, the BX2. This has a sensitivity of 90 dB, is also an 8 Ohm speaker and has power handling of 100W so can output a max. of approx. 104 dB with the same 25 W input. The difference in maximum sound output of the BX1 and BX2 speakers being driven by the 25W amp is (104 dB - 102 dB =) 2 dB, which is about the same volume difference you can expect between the two amps when driving the same speaker. i.e. not huge.
If you want significant volume increase, a more powerful amp (maybe around 60 - 80W RMS/channel into 8 Ohms) will be required or purchase more sensitive/efficient speakers. Or both.