I am concidering purchasing a Low D whistle. I am especially looking at the Alba and MK brands. I would prefer something that does not require an extreme amount of air volume, and finger spacing should be moderate (for a low whistle). Does anyone have an opinion on these models or can sugest another I should be looking into. I would prefer to keep the price under $300 if I can. I would also prefer it to be on the loud side as I often play in a group.
The Susato whistle is loud, and well within your price range.
I bought a Susato Low D and I personally think that it is not loud, considering how loud are all Susato high D whistles. The finger holes are also very much spaced and it is not easy to cover them unless you have big hands.
A solution for this latest problem could be to consider the Susato Dublin low D whistle (the white one) which can be bought with a key for the last finger hole.
Michael Burke’s Viper is within your price range, and fits your other parameters, also. If you lived in Europe, an Overton might be the ticket. However, the exchange rate and shipping costs create a price that is a bit high, for your needs…
The Viper will give you a solid sound in both octaves. However… It would take a truly LOUD low whistle to be heard above a group, when all are playing full-tilt boogie. I have never played such an instrument. I would consider tasteful amplification.
Best.
Byll
There is a limit to how loud a low D whistle will get. The Susato is about as loud as a low D gets, in my experience. I currently play Burke low D’s. Byll’s suggestion to mic the whistle is a good one if you need more volume. If you want a louder sound in a strictly acoustic group, I would suggest looking into a flute.
Feadoggie
Looking into a flute will get you nowhere. You’ll have to put the thing to your lips and blow. 
stay hoopy,
Mike
There are numerous reviews of the many makes of whistles, accessible through the search feature.
I use a Howard. The air requirement is comparatively low, but it is not loud.
I’ve not had the opportunity to try an MK, but I’ve yet to see anything negative about them at all, and indeed, everyone who has reviewed them seems to ‘gush’. If I could afford one of those, I do believe I’d have it.
Good luck with your search and decision.
I’m afraid I may have mislead when I said I needed a loud whistle. We usually do mic our sessions. The sound man/guitar player just prefers to boost his mic over the others 
. I just don’t want a low whistle that is on the quite side as I know many are.
I have heard good things about the Burke although I have also heard it requires a good bit of air. I was hoping someone could give me an idea of how the Alba or MK work for them, and whether they would recommend them.
I am also considering the Sweatheart which I may be able to get for a little over $300
Then any of the whistles mentioned should do fine. Just rattle the sound guy over your mix. There have been numerous reports on all these whistles througout time here, as Annie stated. Read through those posts. Get an idea what you are shooting for, then pull the trigger, buy one and don’t look back.
Feadoggie
I have an older Howard, and it’s better than the two or three more expensive Low Ds I’ve played. Blows easier through the upper octave, and shifts octave better as well. I honestly can’t find a thing to criticise about it after 8 years of playing it.
Oops! Pearoast!
I don’t spend much time on my low Ds, but can speak for the Copeland and Burke Viper - they are the two best I’ve played. Also, Byll, correct me if I’m wrong here. The Viper is surprising in that you get a nice volume back for your air which is not the easy case for some low Ds; I wouldn’t call it loud, but it has ample volume and is quite responsive; also very nicely balanced between the octaves. I don’t find it to have high air requirements at all.
I spent a too short period of time with an MK low D some summers ago at Custy’s in Ennis and loved it; however, I was literally yanked out of the place by wife and daughter to get to some place called the Cliffs of Mohr They were also quite beautiful.
Philo
Whistle isn’t my primary instrument, and when it comes to low whistle, I only dabble. But here are my observations as a dabbler:
The MK, in case you weren’t aware, has a significant wait time, unless you have a lead on getting one 2nd-hand.
My Alba q1 vibe low d (tunable) is not very loud.
My Reviol low D, purchased used from another Chiff and Fippler, is my newest acquisition. It seems significantly louder than my Alba. I like it very much. But, again, I’m only dabbling.
Last time I looked, Doc Jones’ IrishFluteStore had some Reviols for under $300.
I have no experience with Vipers or Susato low Ds.