The tone usually refers to the feeling that comes through in the poem, or the mood that the poem elicits in its readers. For this poem, there is a tone of happiness, contentment, and even a bit of pride. Bradstreet seems very happy in her marriage, content with it, and even a bit boastful or proud of her husband and the bliss they have together. This tone is conveyed through the word choices she uses. Happiness is conveyed through her use of "happy", "love", "heavens", "reward", and the constant repeat of the word "ever", which implies that she is so reassured in her love and happiness that she doesn't feel like it will ever go away. Her contentment and peace really come through as she repeats "ever" over and over again; she is confident and assured.
Her pride and boasting comes through with words like "prize", "mines of gold", "riches", "reward", and "recompense". She feels their love is a prize, compares it to gold, encourages other women to look and admire their love, and considers it worth riches and beyond compare. So, a glow of pride and possibly even boasting is another tone that comes through the poem.
I hope that those thoughts help; good luck!